Frequently blocking climbing unsuitable feature of building (7)
I believe the answer is:
rooftop
'feature of building' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'frequently blocking climbing unsuitable' is the wordplay.
'frequently' becomes 'oft' (oft can mean frequently, eg oft-quoted).
'blocking' indicates putting letters inside (block can mean to plug a gap in something).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'unsuitable' becomes 'poor' (I am not sure about this - if you are sure you should believe this answer much more).
'poor' written backwards gives 'roop'.
'oft' placed into 'roop' is 'ROOFTOP'.
Can you help me to learn more?
(Other definitions for rooftop that I've seen before include "Highest point of dwelling" , "House's apex" , "High point of building (from which to shout?)" , "on the tiles?" , "In place of protest" .)