We return after old king got bigger (4)
I believe the answer is:
grew
'got bigger' is the definition.
(I've seen this before)
'we return after old king' is the wordplay.
'return' is a reversal indicator.
'after' means one lot of letters go next to another.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'we' backwards is 'ew'.
'ew' after 'gr' is 'GREW'.
(Other definitions for grew that I've seen before include "Got larger" , "Got bigger" , "Cultivated'" , "Developed, matured" , "Sprouted" .)