Held up by zoologist, a rare animal (3)
I believe the answer is:
'animal' is the definition.
(rat is a kind of animal)
'held up by zoologist a rare' is the wordplay.
'up by' is a reversal indicator.
'held' indicates a hidden word.
'tar' is hidden within 'zoologist a rare'.
'tar' in reverse letter order is 'RAT'.
(Other definitions for rat that I've seen before include "Domestic rodent" , "one not going down with ship" , "Scumbag" , "Plague-carrying rodent" , "Cad" .)