Suitable to have marten running flat (9)
I believe the answer is:
apartment
'flat' is the definition.
(I know that flat can be written as apartment)
'suitable to have marten running' is the wordplay.
'suitable' becomes 'apt' (apt means appropriate or fitting).
'to have' means one lot of letters goes inside another.
'running' indicates an anagram (the letters run around to different places).
'marten' with letters rearranged gives 'artmen'.
'apt' placed around 'artmen' is 'APARTMENT'.
(Other definitions for apartment that I've seen before include "Accommodation" , "Flat (chiefly US)" , "property" , "quarters" , "19 across" .)