Opener enthusiastic about collecting runs (5)
I believe the answer is:
intro
'opener' is the definition.
(I've seen this before)
'enthusiastic about collecting runs' is the wordplay.
'enthusiastic about' becomes 'into' (into can mean interested in or enthusiastic about).
'collecting' is an insertion indicator.
'runs' becomes 'r' (cricket abbreviation).
'into' enclosing 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "Song's first few bars" , "Beginning (informal)" , "Prelude" , "Opening bars (abbr)" , "Song's opening" .)