A charitable saint saving her first marine creatures (5)
I believe the answer is:
orcas
'marine creatures' is the definition.
(orcas are marine animals)
'a charitable saint saving her first' is the wordplay.
'a charitable saint' becomes 'Dorcas' (St Dorcas).
'saving her first' means to remove the first letter.
'dorcas' with its initial letter taken away is 'ORCAS'.
(Other definitions for orcas that I've seen before include "Killer whales" , "Killers" .)