He's overcome by unusual symptom of cold (5)
I believe the answer is:
rheum
'symptom of cold' is the definition.
(rheum is fluid from eyes)
'he's overcome by unusual' is the wordplay.
'overcome by' means one lot of letters goes inside another.
'unusual' becomes 'rum' (I've seen this before).
'he' placed inside 'rum' is 'RHEUM'.
(Other definitions for rheum that I've seen before include "cold once upon a time" , "Eyes watering" , "Catarrh" , "Watery discharge from eyes or nose" , "Moist discharge from nose or eyes" .)