If lacking in potential, it's improbable (3,6)
I believe the answer is:
not likely
'improbable' is the definition.
(unlikely)
'if lacking in potential it's' is the wordplay.
I cannot really understand how this works, but
'lacking' could be 'no' (eg lacking money means with no money) and 'no' is present in the answer.
'potential' could be 'likely' (synonyms) and 'likely' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for not likely that I've seen is " Toni Kelly confused? Hardly".)