Principal's hard gripping end of strap (6)
I believe the answer is:
staple
'principal's' is the definition.
(thesaurus)
'hard gripping end of strap' is the wordplay.
'hard' becomes 'stale' ('stale' can be similar in meaning to 'hard'**).
'gripping' is an insertion indicator.
'end of' says to take the final letters.
The last letter of 'strap' is 'p'.
'stale' placed around 'p' is 'STAPLE'.
(Other definitions for staple that I've seen before include "important things to eat" , "It holds fast" , "e.g. rice, sweetcorn ..." , "Basic commodity for which demand is constant" , "office item" .)