Done deed? Spot on! (5)
I believe the answer is:
exact
'spot on' is the definition.
(spot on means exactly right)
'done deed?' is the wordplay.
'done' becomes 'ex' (I can't justify this - if you can you should believe this answer much more).
'deed?' becomes 'act' (an action).
'ex'+'act'='EXACT'
Can you help me to learn more?
(Other definitions for exact that I've seen before include "19 [WREST]" , "Precise; extort" , "Absolutely accurate or correct" , "Rigorous" , "Spot on, precise" .)