Jacks drunk Guinness when out of uniform (7)
I believe the answer is:
ensigns
'jacks' is the definition.
(both can mean flags)
'drunk guinness when out of uniform' is the wordplay.
'drunk' indicates anagramming the letters.
'when out of' suggests deleting specific letters.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'guinness' with 'u' taken away is 'ginness'.
'ginness' anagrammed gives 'ENSIGNS'.
(Other definitions for ensigns that I've seen before include "Flags for naval officers" , "Sensing (anag)" , "US naval officers" , "Old soldiers" , "Flag" .)