They line the arteries in time: fat essentially providing blockage (7)
I believe the answer is:
intimae
'they line the arteries' is the definition.
The definition suggests an adverb but the answer is not.
'in time fat essentially providing blockage' is the wordplay.
'in' is an insertion indicator.
'essentially' says to take the centre.
'providing' indicates an anagram.
'blockage' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
The central letter of 'fat' is 'a'.
'time'+'a'='timea'
'timea' with letters rearranged gives 'timae'.
'timae' placed around 'in' is 'INTIMAE'.
Can you help me to learn more?
(Other definitions for intimae that I've seen before include "Inner membranes of blood vessels" , "lining of arteries" , "organ parts" .)