Foot soldiers in cooler ahead of trial (8)
I believe the answer is:
infantry
'foot soldiers' is the definition.
(I've seen this before)
'in cooler ahead of trial' is the wordplay.
'cooler' becomes 'fan' (something which cools).
'ahead of' means one lot of letters go next to another (some letters go in front of others).
'trial' becomes 'try'.
'in'+'fan'+'try'='INFANTRY'
(Other definitions for infantry that I've seen before include "Grunts" , "Foot-soldiers" , "Army unit of foot soldiers" , "Unmounted fighters" , "part of the army" .)