On second thoughts, the horse isn’t tailless (4,5)
I believe the answer is:
then again
'on second thoughts the horse isn't tailless' is the definition.
I can't judge whether this definition defines the answer.
'the horse isn't tailless' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse).
'isn't' becomes 'aint' (I've seen this before).
'tailless' means to remove the last letter (the tail of the word is removed).
'aint' with its final letter taken away is 'ain'.
'the'+'nag'+'ain'='THEN AGAIN'
Can you help me to learn more?
(Other definitions for then again that I've seen before include "From another point of view" , "still" , "however" .)