About to get into swinging? Just be open! (7)
I believe the answer is:
'open' is the definition.
'about to get into swinging? just be' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to get into' is an insertion indicator.
'swinging?' is an anagram indicator.
'justbe' is an anagram of 'subjet'.
'c' going within 'subjet' is 'SUBJECT'.
(Other definitions for subject that I've seen before include "School topic" , "Put down" , "UK citizen?" , "issue" , "Citizenship' at school maybe" .)