Advocating for Mahler's first for concert (4)
I believe the answer is:
prom
'for concert' is the definition.
(abbreviation for promenade concert)
'advocating for mahler's first' is the wordplay.
'advocating' becomes 'pro' (I've seen this in another clue).
'for' says to put letters next to each other.
'mahler's first' becomes 'm' (pst letter of 'mahler').
'pro'+'m'='PROM'
(Other definitions for prom that I've seen before include "Concert - school dance (US)" , "Seafront" , "Classical music concert for which part of the audience stands" , "Albert Hall concert" , "Seaside walk (abbr.)" .)