While covering for the fellow spies (6)
I believe the answer is:
agents
'spies' is the definition.
(secret agents)
'while covering for the fellow' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'covering for' is an insertion indicator.
'the fellow' becomes 'gent' ('gent' can be a synonym of 'fellow').
'as' placed around 'gent' is 'AGENTS'.
(Other definitions for agents that I've seen before include "travelling salesmen" , "Instruments" , "They do business on behalf of others" , "Representatives" , "Sales reps" .)