Substitute Manchester United called on laughably, heads turning (5)
I believe the answer is:
locum
'substitute' is the definition.
(locum is a kind of substitute)
'manchester united called on laughably heads turning' is the wordplay.
'heads' says to take the initial letters.
'turning' says the letters should be written in reverse.
The initial letters of 'manchester united called on laughably' is 'mucol'.
'mucol' backwards is 'LOCUM'.
(Other definitions for locum that I've seen before include "One standing in" , "Relief doctor" , "He fills in" , "Stand-in professional" , "Doctor's temporary replacement" .)