Child's poorly with cold (5)
I believe the answer is:
chill
'cold' is the definition.
(chill can mean cold)
'child's poorly' is the wordplay.
'child' becomes 'ch' (abbreviation for child).
'poorly' becomes 'ill' (synonyms).
'ch'+'ill'='CHILL'
'with' acts as a link.
(Other definitions for chill that I've seen before include "Iciness" , "Unpleasent coldness in the air" , "Make cold; relax" , "Sharp cold" , "Unpleasant coldness in the air" .)