Pressure wore away might (5)
I believe the answer is:
power
'might' is the definition.
(being mighty)
'pressure wore away' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'away' is an anagram indicator (letters move away from their original places).
'wore' is an anagram of 'ower'.
'p'+'ower'='POWER'
(Other definitions for power that I've seen before include "Vigour or energy" , "- - of attorney" , "product" , "military force" , "Military potential" .)