Shock opening of Mahler composition (3)
I believe the answer is:
mop
'shock' is the definition.
(I've seen this before)
'opening of mahler composition' is the wordplay.
'opening of' suggests taking the first letters.
'composition' becomes 'op' (I've seen this in other clues).
The initial letter of 'mahler' is 'm'.
'm'+'op'='MOP'
(Other definitions for mop that I've seen before include "A thick mass of hair on head" , "Clean; mass of hair" , "Floor wiper" , "Wet cleaner" , "Tangle of hair" .)