Opening with Mahler's Fifth generates yawn (4)
I believe the answer is:
gape
'yawn' is the definition.
(I know that yawn can be written as gape)
'opening with mahler's fifth' is the wordplay.
'opening' becomes 'gap' (synonyms).
'with' is a charade indicator (letters next to each other).
'mahler's fifth' becomes 'e' (5th letter of 'mahler').
'gap'+'e'='GAPE'
'generates' is the link.
(Other definitions for gape that I've seen before include "Stare with open mouth in wonder" , "watch in wonder" , "Peer" , "Stare stupidly" , "Look in amazement or stupidity" .)