Constant pain for Arsenal? (5)
I believe the answer is:
cache
'arsenal?' is the definition.
(both can mean a stock of something)
'constant pain' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'pain' becomes 'ache' (ache is a kind of pain).
'c'+'ache'='CACHE'
'for' acts as a link.
(Other definitions for cache that I've seen before include "Concealed store" , "Stockpile" , "Nest egg" , "what things could be stored in" , "Hiding-place for treasure or stores" .)