Diffident when admitting a gee-gee is hairy (6)
I believe the answer is:
shaggy
'hairy' is the definition.
(I've seen this before)
'diffident when admitting a gee-gee' is the wordplay.
'diffident' becomes 'shy' (synonyms).
'when admitting' means one lot of letters goes inside another (inserted letters are admitted or allowed in).
'gee' becomes 'g' (synonyms).
'gee' becomes 'g' (synonyms).
'a'+'g'+'g'='agg'
'shy' going around 'agg' is 'SHAGGY'.
'is' is the link.
(Other definitions for shaggy that I've seen before include "Very hairy, like some dog stories" , "Scooby-Doo's friend" , "(Of fur) long and unkempt" , "Rough-haired" , "Type of dog with story" .)