Subsequently keeping one contralto back for concert (7)
I believe the answer is:
recital
'for concert' is the definition.
(I've seen this before)
'subsequently keeping one contralto back' is the wordplay.
'subsequently' becomes 'later' (subsequently means later on).
'keeping' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'contralto' becomes 'c' (musical abbreviation).
'back' is a reversal indicator.
'i'+'c'='ic'
'later' placed around 'ic' is 'laticer'.
'laticer' backwards is 'RECITAL'.
(Other definitions for recital that I've seen before include "Musical entertainment or a long account" , "Chamber music concert" , "Concert performance" , "statement of facts" , "(Usually solo) concert" .)