Constant pain in store (5)
I believe the answer is:
cache
'store' is the definition.
(cache is a kind of store)
'constant pain' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'pain' becomes 'ache' (ache is a kind of pain**).
'c'+'ache'='CACHE'
'in' is the link.
(Other definitions for cache that I've seen before include "memory" , "(Place in) hidden store" , "Conceal" , "Nest egg" , "Secret source" .)