Pals join line about to cut in (5)
I believe the answer is:
mates
'pals' is the definition.
(I've seen this in another clue)
'join line about to cut in' is the wordplay.
'join line' becomes 'seam' (I've seen this before).
'about' is a reversal indicator.
'cut' means to remove the last letter.
'in' indicates putting letters inside.
'seam' backwards is 'maes'.
'to' with its last letter taken away is 't'.
'maes' placed around 't' is 'MATES'.
(Other definitions for mates that I've seen before include "Pals or spouses" , "Chums" , "Naval officers'" , "Pals or married couples" , "Friends among the ship's officers" .)