If trimming is not an option, try this (it's free):
https://fcpxtemplates.com/sdm_downloads/sc-crossfade-clone/
Apply it to an Adjustment Layer (about 2 seconds long recommended — or however long you need).
What this effect does is take the 1st frame of the storyline over which the AL starts and the Last frame of the storyline over which the AL ends as still images and fades from one to the other.
You **will** have to combine the two adjacent clips into a Compound Clip!!! (The effect will not work otherwise — see below).
You can click and drag the Adjustment Layer over the storyline to be able to determine where the two adjacent clips meet and position the AL there. It is not always possible to see exactly where two clips meet once they're inside a CC, but dragging the AL over the storyline will display where you are in the viewer — like skimming, but Skimming does not have to be turned on.
If you need it for many consecutive clips - just place them all in one CC — option drag copies of the AL to their required locations.
I don't know ***why*** the two adjacent clips need to be "united" in a CC for this to work... FCP seems to split the two clips in feeding them to the template in such a way that the end frames are not accessible until the playhead would physically start to play the second clip... if that makes any sense :P !
HTH