In cooler, test soldiers (8)
I believe the answer is:
infantry
'soldiers' is the definition.
(I've seen this before)
'in cooler test' is the wordplay.
'cooler' becomes 'fan' (I've seen this before).
'test' becomes 'try' ('try' can be a synonym of 'test').
'in'+'fan'+'try'='INFANTRY'
(Other definitions for infantry that I've seen before include "Grunts" , "fighting group" , "Foot-soldiers" , "Men fighting" , "Troops" .)