A tense sort of pilot to be witness (6)
I believe the answer is:
attest
'be witness' is the definition.
(thesaurus)
'a tense sort of pilot' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'sort of pilot' becomes 'test'.
'a'+'t'+'test'='ATTEST'
'to' is the link.
(Other definitions for attest that I've seen before include "Give sworn evidence" , "Affirm by oath" , "Make evident" , "Act as witness" , "Give evidence in a court of law" .)