Ache for unknown merit (5)
I believe the answer is:
yearn
'ache' is the definition.
('yearn' can be a synonym of 'ache')
'unknown merit' is the wordplay.
'unknown' becomes 'y' (algebra).
'merit' becomes 'earn' (I've seen this before).
'y'+'earn'='YEARN'
'for' acts as a link.
(Other definitions for yearn that I've seen before include "Have a great longing" , "Have a craving" , "Have an intense feeling of longing" , "wish" , "Long - pine" .)