Bird female medic observed in expanse of water (9)
I believe the answer is:
sheldrake
'bird' is the definition.
(sheldrake is a kind of bird)
'female medic observed in expanse of water' is the wordplay.
'female' becomes 'she' ('she' refers to a woman).
'medic' becomes 'dr' (doctor).
'observed in' indicates putting letters inside.
'expanse of water' becomes 'lake' (I've seen this before).
'dr' put within 'lake' is 'ldrake'.
'she'+'ldrake'='SHELDRAKE'
(Other definitions for sheldrake that I've seen before include "Large crested diving duck" , "flighty type" , "Bird" , "Diving duck with serrated bill" , "natural swimmer" .)