Rocking etc in old-time musical group (5)
I believe the answer is:
octet
'musical group' is the definition.
(octet is a kind of musical group)
'rocking etc in old-time' is the wordplay.
'rocking' is an anagram indicator.
'in' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'time' becomes 't' (abbreviation).
'etc' anagrammed gives 'cte'.
'o'+'t'='ot'
'cte' placed within 'ot' is 'OCTET'.
(Other definitions for octet that I've seen before include "music group" , "Mendelssohn wrote this" , "Composition for eight musicians" , "Chamber ensemble, a Mendelssohn favourite" , "What takes members up to eighth" .)