Reservations about counterfoil (4)
I believe the answer is:
stub
'counterfoil' is the definition.
(synonyms)
'reservations about' is the wordplay.
'reservations' becomes 'buts' (I've seen this in another clue).
'about' is a reversal indicator.
'buts' written backwards gives 'STUB'.
(Other definitions for stub that I've seen before include "A short stump as of cigar" , "Returned part of a ticket" , "Knock (toe); short remnant" , "Ticket counterfoil" , "Wee bit of pencil, eg," .)