Tying up medic needing oxygen on call (7)
I believe the answer is:
mooring
'tying up' is the definition.
(synonyms)
'medic needing oxygen on call' is the wordplay.
'medic' becomes 'mo' (medical officer).
'needing' says to put letters next to each other.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'on' says to put letters next to each other.
'call' becomes 'ring' (synonyms).
'mo'+'o'+'ring'='MOORING'
(Other definitions for mooring that I've seen before include "Tying up (a boat)" , "Making fast" , "Place to leave a boat" , "Tying up a ship" , "stopping" .)