Right to assist surprise visit (4)
I believe the answer is:
raid
'surprise visit' is the definition.
(I've seen this before)
'right to assist' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to assist' becomes 'aid' (synonyms).
'r'+'aid'='RAID'
(Other definitions for raid that I've seen before include "Sudden short attack" , "Enter a territory and take spoils" , "Sudden swift attack" , "Sally" , "A sudden military attack and withdrawal" .)