Infantile mistake about an extra cut (7)
I believe the answer is:
babyish
'infantile' is the definition.
(I've seen this before)
'mistake about an extra cut' is the wordplay.
'mistake' becomes 'bish' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'an extra cut' becomes 'aby' (I can't justify this - if you can you should give a lot more credence to this answer).
'bish' going around 'aby' is 'BABYISH'.
(Other definitions for babyish that I've seen before include "Typical of new arrival" , "Infantile" , "Immature" .)