Might it be that sister? Poor nun in trouble! (8,7)
I believe the answer is:
relative pronoun
'might it be that' is the definition.
('that' is an example)
'sister? poor nun in trouble' is the wordplay.
'sister?' becomes 'relative' (sister is a kind of relative**).
'in trouble' is an anagram indicator.
'poor'+'nun'='poornun'
'poornun' with letters rearranged gives 'pronoun'.
'relative'+'pronoun'='RELATIVE PRONOUN'
(Other definitions for relative pronoun that I've seen before include "Say which" , "It could be that" , "Maybe what" .)