Explain Pinter's opening in 'Betrayal' (5,3)
I believe the answer is:
spell out
'explain' is the definition.
(thesaurus)
'pinter's opening in betrayal' is the wordplay.
'opening' indicates taking the first letters.
'in' is an insertion indicator.
'betrayal' becomes 'sellout' (sellout is a kind of betrayal).
The first letter of 'pinter' is 'p'.
'p' going into 'sellout' is 'SPELL OUT'.
(Other definitions for spell out that I've seen before include "Temporary release from prison" , "Give precise details" , "writing letters?" , "Explain in detail" .)