Unit at large performing survey (11)
I believe the answer is:
triangulate
'survey' is the definition.
(I've seen this before)
'unit at large performing' is the wordplay.
'performing' is an anagram indicator.
'unit'+'at'+'large'='unitatlarge'
'unitatlarge' anagrammed gives 'TRIANGULATE'.
(Other definitions for triangulate that I've seen before include "Use a method of surveying (an area)" , "Measure in sections" , "to survey position" .)