Strength to take a seat, over 90 outside (9)
I believe the answer is:
intensity
'strength' is the definition.
('intensity' can be a synonym of 'strength')
'a seat over 90 outside' is the wordplay.
I cannot really understand how this works, but
'seat' could be 'sit' ('sit' can be a synonym of 'seat') and 'sit' is located in the answer.
This may be the basis of clue (or it may be nonsense).
'to take' acts as a link.
(Other definitions for intensity that I've seen before include "Strength of feeling or pressure" , "Level, degree or magnitude" , "It's ninety in emotional state" , "Isn't in yet (anag)" , "Depth of feeling or energy" .)