Recall promoters finally brought in rock group in reserve (9)
I believe the answer is:
reminisce
'recall' is the definition.
(I've seen this before)
'promoters finally brought in rock group in reserve' is the wordplay.
'finally' suggests the final letters.
'brought in' is an insertion indicator.
'rock group' becomes 'REM' (REM are a US rock band).
'reserve' becomes 'ice' (both can mean aloofness).
The final letter of 'promoters' is 's'.
'rem'+'in'+'ice'='reminice'
's' put into 'reminice' is 'REMINISCE'.
(Other definitions for reminisce that I've seen before include "Cast mind back over old times" , "Have fond recollections" , "Talk like an oldie" , "reflect" , "Recollect past events" .)