McIlroy’s first club covered constant tiny distance (6)
I believe the answer is:
micron
'tiny distance' is the definition.
(term for a micrometre)
'mcilroy's first club covered constant' is the wordplay.
'mcilroy's first' becomes 'm' (1st letter of 'mcilroy').
'club' becomes 'iron' (type of golf club).
'covered' is an insertion indicator.
'constant' becomes 'c' (mathematical abbreviation).
'iron' placed around 'c' is 'icron'.
'm'+'icron'='MICRON'
(Other definitions for micron that I've seen before include "One-millionth of a metre" , "One millionth of 3 Across" , "Micrometre" , "Small length" , "One millionth of 1 metre" .)