Learn in the army (4)
I believe the answer is:
hear
'learn' is the definition.
(I've seen this before)
'in the army' is the wordplay.
'in' indicates a hidden word.
'HEAR' can be found hidden inside 'the army'.
(Other definitions for hear that I've seen before include "Perceive audibly" , "Pay attention" , "Perceive sound" , "Perceive aurally" , "Discover aurally" .)