Learn too little to be reserve actor? (10)
I believe the answer is:
understudy
'learn' is the definition.
(I know that understudy is a more specific form of the action learn)
'too little to be reserve actor?' is the wordplay.
'too little' becomes 'under' (under a desired amount).
'to be reserve actor?' becomes 'study' (I can't justify this - if you can you should give a lot more credence to this answer).
'under'+'study'='UNDERSTUDY'
(Other definitions for understudy that I've seen before include "Stage stand-in" , "emergency role" , "Replacement" , "one's hoping to get on" , "One prepared to take over from leading actor" .)