Lake to merit study (5)
I believe the answer is:
learn
'study' is the definition.
(I've seen this before)
'lake to merit' is the wordplay.
'lake' becomes 'l'.
'to merit' becomes 'earn' (I've seen this before).
'l'+'earn'='LEARN'
(Other definitions for learn that I've seen before include "teach illiterate" , "Get to know" , "Pick up" , "Study - find out" , "Study or digest" .)