Firmly fix hedge, trimming the top (5)
I believe the answer is:
rivet
'firmly fix' is the definition.
'hedge trimming the top' is the wordplay.
'hedge' becomes 'privet' (I've seen this before).
'trimming the top' means to remove the first letter.
'privet' with its first letter removed is 'RIVET'.
(Other definitions for rivet that I've seen before include "fix" , "Captivate" , "Fasten -- engross" , "Arrest" , "Bolt in iron" .)