Pick up head of lettuce to take home (5)
I believe the answer is:
Here is my best explanation:
'pick up' is the definition.
('learn' can be a synonym of 'pick up')
'head of lettuce to take home' is the subsidiary indication.
'head of' indicates taking the first letters.
'to take home' becomes 'earn'.
The first letter of 'lettuce' is 'l'.
(Other definitions for learn that I've seen before include "Pick up", "Master", "Gain knowledge or skills", "Become aware of", "Study - find out", "Study - discover", "Study or digest", "what pupils do in schools", "Discover - study" and "Renal (anag)".)