Speaker welcoming physical exercise as runner (8)
I believe the answer is:
operator
'runner' is the definition.
(thesaurus)
'speaker welcoming physical exercise' is the wordplay.
'speaker' becomes 'orator' (orator is a kind of speaker).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'physical exercise' becomes 'pe'.
'orator' enclosing 'pe' is 'OPERATOR'.
'as' is the link.
(Other definitions for operator that I've seen before include "One working a machine" , "Poor rate (anag.)" , "Machinist -- or surgeon?" , "Controller, of telephone say" , "Poor rate for a businessman" .)