Effusive fellow about to pipe down (5)
I believe the answer is:
gushy
'effusive' is the definition.
(I know that effusive can be written as gushy)
'fellow about to pipe down' is the wordplay.
'fellow' becomes 'guy' (both can mean a man).
'about' means one lot of letters goes inside another.
'to pipe down' becomes 'sh' (both can mean 'be quiet').
'guy' enclosing 'sh' is 'GUSHY'.
(Other definitions for gushy that I've seen before include "Effusively sentimental" , "Overly effusive" .)