Medical officer to exploit timid creature (5)
I believe the answer is:
mouse
'timid creature' is the definition.
(I've seen this before)
'medical officer to exploit' is the wordplay.
'medical officer' becomes 'MO' (abbreviation).
'to exploit' becomes 'use' (use is a kind of exploit).
'mo'+'use'='MOUSE'
(Other definitions for mouse that I've seen before include "Shy individual" , "Timid creature" , "Domestic rodent" , "computer operator" , "Wimp" .)