Ache for unknown merit (5)
I believe the answer is:
yearn
'ache for' is the definition.
(I've seen this in another clue)
'unknown merit' is the wordplay.
'unknown' becomes 'y' (algebra).
'merit' becomes 'earn' (I've seen this in another clue).
'y'+'earn'='YEARN'
(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" .)