Mum entertained by Jagger et al performing — one getting into rock? (10)
I believe the answer is:
stonemason
'one getting into rock?' is the definition.
'mum entertained by jagger et al performing' is the wordplay.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'entertained by' is an insertion indicator (I've seen this in other clues).
'jagger et al' becomes 'stones' (Mick Jagger and The Rolling Stones).
'performing' becomes 'on' (I've seen this in other clues).
'ma' placed into 'stones' is 'stonemas'.
'stonemas'+'on'='STONEMASON'
(Other definitions for stonemason that I've seen before include "Rock craftsman" , "Construction worker" , "Monumental worker" , "chipper?" , "Architectural carver" .)