Not only post, presumably, that’s slow (7)
I believe the answer is:
andante
'slow' is the definition.
(similar in meaning)
'not only post presumably' is the wordplay.
I cannot really understand how this works, but
'not' could be 'nt' (synonyms) and 'nt' is found within the answer.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for andante that I've seen before include "at a gentle pace" , "progressing slowly" , "moving unhurriedly" , "Moderately slow movement" , "For players, rather slow" .)