Male casualty in US I look after (9)
I believe the answer is:
chaperone
'look after' is the definition.
(I've seen this before)
'male casualty in us i' is the wordplay.
'male' becomes 'chap' (chap is a kind of male).
'casualty in us' becomes 'ER' (emergency room, US equivalent to casualty dept.).
'i' becomes 'one' (Roman numeral for one).
'chap'+'er'+'one'='CHAPERONE'
(Other definitions for chaperone that I've seen before include "Young lady's protector" , "attendant" , "One overseeing" , "Companion" , "Guardian" .)