A person of habit, male running kilometres (4)
I believe the answer is:
monk
'a person of habit' is the definition.
(a monk's robe is called a habit)
'male running kilometres' is the wordplay.
'male' becomes 'm' (common abbreviation).
'running' becomes 'on' (eg when a machine is running it is on).
'kilometres' becomes 'k'.
'm'+'on'+'k'='MONK'
(Other definitions for monk that I've seen before include "Male member of religious order" , "One vowed" , "Thelonious - - (jazz)" , "Friar" , "prayer?" .)