Hard to grip top of plastic fastener (6)
I believe the answer is:
staple
'fastener' is the definition.
(staple is a kind of fastener)
'hard to grip top of plastic' is the wordplay.
'hard' becomes 'stale' ('stale' can be similar in meaning to 'hard'**).
'to grip' means one lot of letters goes inside another.
'top of' says to take the initial letters.
The initial letter of 'plastic' is 'p'.
'stale' going around 'p' is 'STAPLE'.
(Other definitions for staple that I've seen before include "Fundamental" , "Fastening wire" , "Secure" , "A wire paper fastener for plates" , "(Of food) necessary and important" .)