Messenger caught in strange union (6)
I believe the answer is:
nuncio
'messenger' is the definition.
(thesaurus)
'caught in strange union' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'strange' indicates anagramming the letters.
'union' anagrammed gives 'nunio'.
'c' inserted inside 'nunio' is 'NUNCIO'.
(Other definitions for nuncio that I've seen before include "Papal envoy" , "See man" , "Papal ambassador" , "Sent by the Pope" , "Papal messenger" .)