Agents gripped by first half of writer's supplement (7)
I believe the answer is:
augment
'supplement' is the definition.
(I've seen this before)
'agents gripped by first half of writer's' is the wordplay.
'agents' becomes 'gmen' (G-man is an example).
'gripped by' indicates putting letters inside.
'first' becomes 'a'.
'half of writer' becomes 'ut' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'ut'='aut'
'gmen' put inside 'aut' is 'AUGMENT'.
Can you help me to learn more?
(Other definitions for augment that I've seen before include "Swell" , "boost" , "Grow - intensify" , "Make larger" , "Enlarge or increase" .)