Dense fog — Central tube station first to get disrupted (6)
I believe the answer is:
obtuse
'dense' is the definition.
('obtuse' can be a synonym of 'dense')
'fog central tube station first to get disrupted' is the wordplay.
'central' means to look at the middle letters.
'first' suggests taking the first letters.
'to get disrupted' indicates anagramming the letters.
The middle of 'fog' is 'o'.
The first letter of 'station' is 's'.
'tube'+'s'='tubes'
'tubes' anagrammed gives 'btuse'.
'o'+'btuse'='OBTUSE'
(Other definitions for obtuse that I've seen before include "angle, perhaps" , "Angle between 90 and 180 degrees" , "not acute?" , "Slow-witted; type of angle" , "Dull, non-perceptive" .)