Agitation after leading pair’s decamped, note, a catalyst (6)
I believe the answer is:
enzyme
'catalyst' is the definition.
(I know that enzyme is a type of catalyst)
'agitation after leading pair's decamped note a' is the wordplay.
I cannot really understand how this works, but
'note' could be 'me' (musical note as in do-re-me) and 'me' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for enzyme that I've seen before include "Protein produced by living cells" , "Substance produced by living cells" , "Protein substance acting as a catalyst" , "One causing a reaction" , "Protein product of living cells" .)