Afraid to spill RAF fuel (7)
I believe the answer is:
fearful
'afraid' is the definition.
(similar in meaning)
'spill raf fuel' is the wordplay.
'spill' is an anagram indicator (letters spilled or knocked around).
'raf'+'fuel'='raffuel'
'raffuel' is an anagram of 'FEARFUL'.
'to' is the link.
(Other definitions for fearful that I've seen before include "Apprehensive, worried" , "wary" , "Frightening" , "awful!" , "Scared" .)