A gee-gee restricted by fat - one falling behind (7)
I believe the answer is:
laggard
'one falling behind' is the definition.
(I've seen this before)
'a gee-gee restricted by fat' is the wordplay.
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'restricted by' is an insertion indicator (I've seen this in other clues).
'fat' becomes 'lard' (I've seen this before).
'a'+'g'+'g'='agg'
'agg' placed inside 'lard' is 'LAGGARD'.
(Other definitions for laggard that I've seen before include "slowcoach" , "one going slowly" , "Drag gal around for one who falls behind" , "slower than expected" , "Slow mover" .)