Sovereign tucked into fish in lounge (4)
I believe the answer is:
idle
'in lounge' is the definition.
(I've seen this before)
'sovereign tucked into fish' is the wordplay.
'sovereign' becomes 'l' (sovereign can mean pound and therefore L).
'tucked into' means one lot of letters goes inside another.
'fish' becomes 'ide' (I've seen this before**).
'l' placed within 'ide' is 'IDLE'.
(Other definitions for idle that I've seen before include "Unoccupied or not in use" , "Dormant" , "Mike" , "Tick over like engine" , "staying in bed all day?" .)