Fast old speed (5)
I believe the answer is:
lento
'fast old speed' is the definition.
Both the definition and answer are adverbs. Maybe you can see an association between them that I can't see?
'fast old' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'lent'+'o'='LENTO'
Can you help me to learn more?
(Other definitions for lento that I've seen before include "(When playing music) slowly" , "Slow (mus.)" , "Scored slowly" , "How one scores slowly" , "It's slower than adagio" .)