Apply fixed rate, entering PIN (7)
I believe the answer is:
pertain
'apply' is the definition.
(I know that apply is a kind of pertain)
'fixed rate entering pin' is the wordplay.
'fixed' is an anagram indicator.
'entering' is an insertion indicator.
'rate' with letters rearranged gives 'erta'.
'erta' put within 'pin' is 'PERTAIN'.
(Other definitions for pertain that I've seen before include "Have relevance" , "Relate (to), apply (to)" , "Be relevant, or belong as part" , "Refer" , "Painter inclined to be relevant" .)