Drug for unknown adult having cold (6)
I believe the answer is:
prozac
'drug' is the definition.
(I've seen this before)
'for unknown adult having cold' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'unknown' becomes 'z' (represents an unknown value in algebra).
'adult' becomes 'a' (abbreviation).
'having' says to put letters next to each other.
'cold' becomes 'C' (eg on taps).
'pro'+'z'+'a'+'c'='PROZAC'
(Other definitions for prozac that I've seen before include "Controversial antidepressant" , "Upper" , "medication" , "Common antidepressant drug" , "Anti-depressant trade mark" .)