Goes on about horses (4)
I believe the answer is:
nags
'horses' is the definition.
(I know this)
'goes on about' is the wordplay.
I cannot quite see how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is found in the answer.
The remaining letters 'ngs' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for nags that I've seen before include "Annoys" , "Scolds - horses" , "Scolds continually" , "Pesters" , "Constantly reproaches and complains" .)