Dirty location near Heathrow almost changed finally (7)
I believe the answer is:
stained
'dirty' is the definition.
(thesaurus)
'location near heathrow almost changed finally' is the wordplay.
'location near heathrow' becomes 'staines' (Staines-upon-Thames).
'almost' means to remove the last letter (most of the word but not all of it).
'finally' says to take the final letters.
The last letter of 'changed' is 'd'.
'staines' with its final letter removed is 'staine'.
'staine'+'d'='STAINED'
(Other definitions for stained that I've seen before include "___ glass" , "Like many church windows" , "Marked, discoloured" , "Church feature - side sold was wanting (anag)" , "Soiled" .)