Mad medic on small lorry (10)
I believe the answer is:
moonstruck
'mad' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'medic on small lorry' is the wordplay.
'medic' becomes 'mo' (medical officer).
'small' becomes 's' (abbreviation - e.g. clothes size).
'lorry' becomes 'truck' (I've seen this before).
'mo'+'on'+'s'+'truck'='MOONSTRUCK'
Can you help me to learn more?
(Other definitions for moonstruck that I've seen before include "Lunatic" , "Crazy" .)