Gag the man to guard against losing secrets initially (5)
I believe the answer is:
heave
'gag' is the definition.
(both can mean to go to throw up)
'man to guard against losing secrets initially' is the wordplay.
I cannot really understand how this works, but
'man' could be 'ave' and 'ave' is found in the answer.
The remaining letters 'he' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'the' acts as a link.
(Other definitions for heave that I've seen before include "Produce (a sigh); haul" , "Retch - lift using much effort" , "gag" , "Gasp" , "cast like this" .)