Worry, seeing gee-gee entering river (6)
I believe the answer is:
niggle
'worry' is the definition.
(niggling is a kind of worrying)
'gee-gee entering river' is the wordplay.
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'entering' indicates putting letters inside.
'river' becomes 'nile' (Nile is an example).
'g'+'g'='gg'
'gg' going within 'nile' is 'NIGGLE'.
'seeing' is the link.
(Other definitions for niggle that I've seen before include "Grumble" , "Persistant annoyance" , "Persistently criticise over trifles" , "Carp" , "Annoy slightly" .)