Show mattered, son performing (11)
I believe the answer is:
demonstrate
'show' is the definition.
(synonyms)
'mattered son performing' is the wordplay.
'performing' indicates anagramming the letters (the letters move around or 'perform').
'mattered'+'son'='matteredson'
'matteredson' with letters rearranged gives 'DEMONSTRATE'.
(Other definitions for demonstrate that I've seen before include "Show or display" , "English floozies returning, led by villainous character" , "March" , "Review" , "Prove, show" .)