People failing to reach potential may possess mountains full of energy (5-4-5)

'people failing to reach potential' is the definition.
(people who might have been successful)

'may possess mountains full of energy' is the wordplay.
'may' becomes 'might' (synonyms).
'possess' becomes 'have' (synonyms).
'mountains' becomes 'bens' (as in Ben Nevis in Scotland).
'full of' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mca).
'bens' going around 'e' is 'beens'.
'might'+'have'+'beens'='MIGHT-HAVE-BEENS'