It may help your headache when the parking's poor (4)
I believe the answer is:
pill
'it may help your headache' is the definition.
The definition suggests a singular noun which matches the answer.
'the parking's poor' is the wordplay.
'the parking' becomes 'p'.
'poor' becomes 'ill' (I've seen this before).
'p'+'ill'='PILL'
'when' acts as a link.
(Other definitions for pill that I've seen before include "Form of medicine" , "Dreadful person" , "may be the cure" , "Medical tablet" , "Medicinal pellet" .)