Pair messing with the head of Dean Swift (5)
I believe the answer is:
rapid
'swift' is the definition.
(I've seen this in another clue)
'pair messing with the head of dean' is the wordplay.
'messing with' indicates anagramming the letters (I've seen 'mess' mean this).
'the head of' suggests taking the first letters.
The first letter of 'dean' is 'd'.
'pair' with its letters anagrammed with 'd' makes 'RAPID'.
(Other definitions for rapid that I've seen before include "Brisk" , "Zippy" , "Prompt" , "Fast, speedy" , "Showing pace" .)