While covering for the fellow spies (6)
I believe the answer is:
agents
'spies' is the definition.
(I've seen this before)
'while covering for the fellow' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'covering for' means one lot of letters goes inside another.
'the fellow' becomes 'gent' (synonyms).
'as' enclosing '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" .)