Queen to withdraw for this reason? (4)
I believe the answer is:
ergo
'for this reason?' is the definition.
(thesaurus)
'queen to withdraw' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'to withdraw' becomes 'go'.
'er'+'go'='ERGO'
(Other definitions for ergo that I've seen before include "as a consequence" , "It follows" , "'Cogito, ---- sum'" , "Therefore (Latin)" , "so philosopher wrote in Latin" .)