Rubbish musical about Heart of Midlothian (3,3)
I believe the answer is:
hot air
'rubbish' is the definition.
'musical about heart of midlothian' is the wordplay.
'musical' becomes 'hair' (I've seen this before).
'about' means one lot of letters goes inside another.
'heart of' says to take the centre.
The middle of 'midlothian' is 'ot'.
'hair' placed around 'ot' is 'HOT-AIR'.
(Other definitions for hot air that I've seen before include "nothing worth listening to" , "Bluster" , "over the Sahara?" , "Bombast or balloon filler" , "Gas" .)