On the other hand, the horse is not tailless (4,5)
I believe the answer is:
then again
'on the other hand' is the definition.
(synonyms)
'the horse is not tailless' is the wordplay.
'horse' becomes 'nag' (term for an old horse).
'is not' becomes 'ain't' ('ain't' is a non-standard form of 'isn't').
'tailless' means to remove the last letter (the tail of the word is removed).
'aint' with its last letter taken away is 'ain'.
'the'+'nag'+'ain'='THEN AGAIN'
(Other definitions for then again that I've seen before include "still" , "From another point of view" , "however" .)