Go in with specialist on return (5)
I believe the answer is:
enter
'go in' is the definition.
(as in entering a building)
'with specialist on return' is the wordplay.
'with' means one lot of letters go next to another.
'specialist' becomes 'ent' (Ear, Nose and Throat specialist).
'on' becomes 're' (regarding).
'return' shows that the letters should be reversed in order.
're' in reverse letter order is 'er'.
'ent'+'er' is 'ENTER'.
(Other definitions for enter that I've seen before include "Go in - set down" , "cross the threshold" , "Infiltrate" , "Fruit drink" , "Come/go in" .)