Concert performance's beginning on time, on the dot (6)
I believe the answer is:
prompt
'on the dot' is the definition.
(I've seen this before)
'concert performance's beginning on time' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'beginning' suggests taking the first letters.
'on' means one lot of letters go next to another.
'time' becomes 't'.
The initial letter of 'performances' is 'p'.
'prom'+'p'+'t'='PROMPT'
(Other definitions for prompt that I've seen before include "Spurs do this" , "Remind (actor) of lines" , "Bring about; on time" , "Instantaneous" , "Immediate - supply actor's lines" .)