The pound is to a certain amount flexible (7)
I believe the answer is:
lissome
'flexible' is the definition.
(I've seen this before)
'the pound is to a certain amount' is the wordplay.
'the pound' becomes 'L' (the pound sign is based on an L).
'to a certain amount' becomes 'some' (I've seen this in another clue).
'l'+'is'+'some'='LISSOME'
(Other definitions for lissome that I've seen before include "Oil mess (anag)" , "So smile, being agile and lithe" , "pliable" , "Thin - supple - graceful" , "flexible" .)