Following intelligence, top spy infiltrates gathering on the quiet (10)
I believe the answer is:
informally
'following intelligence top spy infiltrates gathering on the quiet' is the definition.
I can't tell whether this defines the answer.
'intelligence top spy infiltrates gathering on the quiet' is the wordplay.
I cannot quite see how this works, but
'intelligence' could be 'info' (I've seen this before) and 'info' is present in the answer.
'spy' could be 'm' ('M' is James Bond's boss) and 'm' is located in the answer.
'gathering' could be 'rally' (rally is a kind of gathering) and 'rally' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for informally that I've seen before include "off the record" , "in relaxed fashion" , "in relaxed manner" .)