Ran causes uncertainly to gain self-confidence (9)
I believe the answer is:
assurance
'self-confidence' is the definition.
(I know that self-confidence can be written as assurance)
'ran causes uncertainly to gain' is the wordplay.
'uncertainly' indicates an anagram.
'to gain' is an insertion indicator.
'causes' anagrammed gives 'assuce'.
'ran' inserted within 'assuce' is 'ASSURANCE'.
(Other definitions for assurance that I've seen before include "Confidence, composure" , "Confidence, certainty" , "Confidence; security" , "Confidence (in abilities)" , "Guarantee - belief in oneself" .)