In run-down environment, finally keep fast (6)
I believe the answer is:
speedy
'fast' is the definition.
(similar in meaning)
'in run-down environment finally keep' is the wordplay.
'in' means one lot of letters goes inside another.
'run-down environment' becomes 'seedy' (I can't explain this - if you can you should give a lot more credence to this answer).
'finally' indicates one should take the final letters.
The final letter of 'keep' is 'p'.
'seedy' going around 'p' is 'SPEEDY'.
Can you help me to learn more?
(Other definitions for speedy that I've seen before include "in a fast car?" , "Done quickly" , "'Fast, without delay (6)'" , "in a hurry" , "Moving quickly" .)