Plant or animal doctor on cuttlefish shell (5)
I believe the answer is:
vetch
'plant' is the definition.
(vetch is a kind of plant)
'animal doctor on cuttlefish shell' is the wordplay.
'animal doctor' becomes 'vet' (veterinarian).
'on' is a charade indicator (letters next to each other).
'shell' suggests removing the centre (only the shell of the word is left).
'cuttlefish' with its centre taken out is 'ch'.
'vet'+'ch'='VETCH'
'or' acts as a link.
(Other definitions for vetch that I've seen before include "Weed" , "Tare" , "Climbing plant of the pea family, cultivated as fodder" , "Climber" , "Wild plant with purple flowers" .)