Hide onset of chronic pain (5)
I believe the answer is:
cache
'hide' is the definition.
(I've seen this in another clue)
'onset of chronic pain' is the wordplay.
'onset of' indicates taking the first letters.
'pain' becomes 'ache' (ache is a kind of pain).
The first letter of 'chronic' is 'c'.
'c'+'ache'='CACHE'
(Other definitions for cache that I've seen before include "Secret source" , "Nest egg" , "Secret hiding place" , "Collection of hidden things" , "Secret store of valuables" .)