From then on centre of area is not affected (7)
I believe the answer is:
sincere
'not affected' is the definition.
(I've seen this in another clue)
'from then on centre of area' is the wordplay.
'from then on' becomes 'since' (I've seen this in another clue).
'centre of' means to look at the middle letters.
The centre of 'area' is 're'.
'since'+'re'='SINCERE'
'is' acts as a link.
(Other definitions for sincere that I've seen before include "Lacking in deceit" , "Candid" , "Unaffected" , "devout" , "Perfect" .)