Reserve the leading part for Merman (5)
I believe the answer is:
ethel
'merman' is the definition.
(I've seen this before)
'reserve the leading part' is the wordplay.
'part' says the answer is hidden in the clue.
'ETHEL' is hidden in the letters of 'reserve the leading'.
'for' is the link.
(Other definitions for ethel that I've seen before include "girl" , "Lady" , "Dame - Smyth, Eng. composer, suffragette" , "Lethe (anag) -- Barrymore or Merman" , "Trying not to be seen" .)