Discharge high explosive during game with Mike (5)
I believe the answer is:
rheum
'discharge' is the definition.
(I know that rheum is a type of discharge)
'high explosive during game with mike' is the wordplay.
'high explosive' becomes 'he'.
'during' means one lot of letters goes inside another.
'game' becomes 'RU' (rugby union).
'with' means one lot of letters go next to another.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'he' going into 'ru' is 'rheu'.
'rheu'+'m'='RHEUM'
(Other definitions for rheum that I've seen before include "nasal discharge" , "sign of illness" , "Rhubarb genus; a head cold" , "drips from the nose" , "Tears of a cold" .)