Enthusiastic about inviting queen for opening bars (5)
I believe the answer is:
intro
'opening bars' is the definition.
(I've seen this before)
'enthusiastic about inviting queen' is the wordplay.
'enthusiastic about' becomes 'into' (I've seen this before).
'inviting' is an insertion indicator.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'into' enclosing 'r' is 'INTRO'.
'for' acts as a link.
(Other definitions for intro that I've seen before include "Beginning passage" , "First bars of a piece of popular music" , "Jazz piece" , "Preamble" , "Lead-in" .)