In the morning, doctor collected shells (4)
I believe the answer is:
ammo
'shells' is the definition.
(I've seen this in another clue)
'in the morning doctor collected' is the wordplay.
'in the morning' becomes 'am' (AM on a clock).
'doctor' becomes 'mo' (medical officer).
'collected' means one lot of letters go next to another.
'am'+'mo'='AMMO'
(Other definitions for ammo that I've seen before include "Explosive devices" , "Slugs perhaps" , "Bullets (abbrev.)" , "Contents of magazine" , "What's fired from guns (abbr)" .)