Supports children playing pianos (including Mike) (9)
I believe the answer is:
champions
'supports' is the definition.
(champion can mean to support or recommend)
'children playing pianos including mike' is the wordplay.
'children' becomes 'ch' (genealogical abbreviation).
'playing' is an anagram indicator.
'including' is an insertion indicator.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'pianos' is an anagram of 'apions'.
'apions' going around 'm' is 'ampions'.
'ch'+'ampions'='CHAMPIONS'
(Other definitions for champions that I've seen before include "best ones" , "Defends" , "They win first place in competitions" , "Pinch Amos to find successful competitors" , "Winners in competition" .)