Favourite bird holds firm to a slip (9)
I believe the answer is:
petticoat
'slip' is the definition.
(I've seen this before)
'favourite bird holds firm to a' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'bird' becomes 'tit' (tit is a kind of bird).
'holds' means one lot of letters goes inside another.
'firm' becomes 'co' (a firm is a company).
'to' is a charade indicator (letters next to each other).
'co'+'a'='coa'
'tit' enclosing 'coa' is 'ticoat'.
'pet'+'ticoat'='PETTICOAT'
(Other definitions for petticoat that I've seen before include "Female undergarment" , "London Lane" , "Woman's underskirt" , "Slip" , "Woman's undergarment" .)