Man’s one of four keeping tail on president’s assassins (3,5)
I believe the answer is:
hit squad
'assassins' is the definition.
(a hit squad is a group of assassins)
'man's one of four keeping tail on president's' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'man' could be 'ad' and 'ad' is found within the answer.
This explanation may well be incorrect...
(Other definitions for hit squad that I've seen before include "assassins" , "Criminal gang" , "collection of offers?" .)