Promise fool to turn crane around (9)
I believe the answer is:
assurance
'promise' is the definition.
(assurance is a kind of promise)
'fool to turn crane around' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'to turn' becomes 'u' (a 'U-turn' is a kind of turn).
'around' is an anagram indicator.
'crane' with letters rearranged gives 'rance'.
'ass'+'u'+'rance'='ASSURANCE'
(Other definitions for assurance that I've seen before include "what may follow life" , "Confidence, certainty" , "Certainty; life insurance" , "Confidence; security" , "Guarantee - belief in oneself" .)