Friends join line the wrong way, having to cut in (5)
I believe the answer is:
mates
'friends' is the definition.
(I know this)
'join line the wrong way having to cut in' is the wordplay.
'join line' becomes 'seam' (I've seen this before).
'the wrong way' shows that the letters should be reversed in order.
'having' is an insertion indicator.
'to cut in' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'seam' back-to-front is 'maes'.
'maes' placed around 't' is 'MATES'.
Can you help me to learn more?
(Other definitions for mates that I've seen before include "conclusive moves on board" , "Naval officers'" , "Friends among the ship's officers" , "Pals, pairs" , "Chums" .)