Horse keeps dry round the bend (5)
I believe the answer is:
batty
'round the bend' is the definition.
(thesaurus)
'horse keeps dry' is the wordplay.
'horse' becomes 'bay' (I've seen this before).
'keeps' means one lot of letters goes inside another.
'dry' becomes 'tt' (abbreviation for teetotal).
'bay' going around 'tt' is 'BATTY'.
(Other definitions for batty that I've seen before include "Dotty!" , "Seemingly descriptive of vampire" , "Eccentric" , "Crazy (sl.)" , "not sound" .)