Getting on a gee-gee outside home (5)
I believe the answer is:
aging
'getting on' is the definition.
(I've seen this before)
'a gee-gee outside home' is the wordplay.
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'outside' means one lot of letters goes inside another.
'home' becomes 'in' ('he's home' can mean 'he's in').
'g'+'g'='gg'
'gg' going around 'in' is 'ging'.
'a'+'ging'='AGING'
(Other definitions for aging that I've seen before include "Growing old in the US" , "in decline" , "Maturing" , "development" , "Becoming more grown-up" .)