Oppressive main space (6)
I believe the answer is:
leaden
'oppressive' is the definition.
(I've seen this before)
'main space' is the wordplay.
'main' becomes 'lead' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'lead'+'en'='LEADEN'
Can you help me to learn more?
(Other definitions for leaden that I've seen before include "Heavy - overcast" , "Depressing - heavy" , "Gloomy - slow-moving" , "Oppressive" , "Heavy and slow, as if weighted down with heavy metal" .)