Strength and pressure wore off (5)
I believe the answer is:
power
'strength' is the definition.
(power is a kind of strength)
'pressure wore off' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'off' is an anagram indicator.
'wore' anagrammed gives 'ower'.
'p'+'ower'='POWER'
'and' acts as a link.
(Other definitions for power that I've seen before include "Possession of controlling influence" , "Political authority" , "Three to the third - is twenty-seven" , "Ability to act or physical might" , "Rate of doing work" .)