Additional folk joining in clarinet playing (11)
I believe the answer is:
incremental
'additional' is the definition.
(increments are regular additions)
'folk joining in clarinet playing' is the wordplay.
'folk' becomes 'men' (both can mean people).
'joining in' indicates putting letters inside.
'playing' indicates an anagram.
'clarinet' is an anagram of 'incretal'.
'men' placed within 'incretal' is 'INCREMENTAL'.
(Other definitions for incremental that I've seen before include "Advancing by small amounts" , "Changing by stages" , "more gradually?" , "Like a rise in" , "Changing by degrees" .)