Fast? Old and slow (5)
I believe the answer is:
lento
'slow' is the definition.
('lento' can be similar in meaning to 'slow')
'fast? old' is the wordplay.
'fast?' becomes 'lent' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lent'+'o'='LENTO'
'and' is the link.
(Other definitions for lento that I've seen before include "Slow musically" , "(Of tempo) slow" , "Slow passage" , "Slow (mus.)" , "Old sayings" .)