If you’re lacking confidence, it’s not safe (8)
I believe the answer is:
insecure
'if you're lacking confidence' is the definition.
I can't tell whether this defines the answer.
'it's not safe' is the wordplay.
'it's not' becomes 'in' (this could be a standard abbreviation I've not previously seen).
'safe' becomes 'secure' (synonyms).
'in'+'secure'='INSECURE'
Can you help me to learn more?
(Other definitions for insecure that I've seen before include "Sinecure arranged for the nervous" , "Not adequately protected" , "Timid" , "Lacking self-confidence or assurance" , "Sinecure for those lacking in confidence" .)