Ask about mission (7)
I believe the answer is:
request
'ask' is the definition.
('request' can be a synonym of 'ask')
'about mission' is the wordplay.
'about' becomes 're' (regarding).
'mission' becomes 'quest' (I've seen this before).
're'+'quest'='REQUEST'
(Other definitions for request that I've seen before include "bidding" , "Queer St (anag.)" , "Appeal" , "Ask for" , "Application" .)