Commercial currency exchange is slow (6)
I believe the answer is:
adagio
'is slow' is the definition.
(similar in meaning)
'commercial currency exchange' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'currency exchange' becomes 'agio' (I can't justify this - if you can you should give a lot more credence to this answer).
'ad'+'agio'='ADAGIO'
Can you help me to learn more?
(Other definitions for adagio that I've seen before include "(Piece to be played) slowly" , "Slow musical passage" , "Slow piece of music" , "Movement in slow time" , "Slow and graceful, musically" .)