Regularly allow club master to provide substitute (5)
I believe the answer is:
locum
'substitute' is the definition.
(locum can mean a substitute)
'regularly allow club master' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'master' becomes 'M' (common abbreviation eg MA, Master of Art).
'allow'+'club'='allowclub'
The alternate letters of 'allowclub' are 'locu'.
'locu'+'m'='LOCUM'
'to provide' is the link.
(Other definitions for locum that I've seen before include "Professional who stands in for another" , "Person filling in" , "Temporary substitute" , "Stand-in GP" , "Stand-in physician" .)