Unexpectedly, 'retain VAR' is in report! (9)
I believe the answer is:
narrative
'report' is the definition.
(narrative is a kind of report)
'unexpectedly retain var' is the wordplay.
'unexpectedly' indicates an anagram.
'retain'+'var'='retainvar'
'retainvar' with letters rearranged gives 'NARRATIVE'.
'is in' is the link.
(Other definitions for narrative that I've seen before include "Popeye's vegetable" , "Story recounting events" , "Report" , "Chronicle" , "Story line" .)