Call to collect sick nun? That's dashing! (7)
I believe the answer is:
running
'that's dashing' is the definition.
(thesaurus)
'call to collect sick nun?' is the wordplay.
'call' becomes 'ring' (telephone call).
'to collect' is an insertion indicator (inserted letters are collected by others).
'sick' indicates anagramming the letters.
'nun' is an anagram of 'unn'.
'ring' enclosing 'unn' is 'RUNNING'.
(Other definitions for running that I've seen before include "healthy activity?" , "administration" , "Jogging" , "in action" , "White City event" .)