Porter regularly going over lovely, small building (6)
I believe the answer is:
prefab
'building' is the definition.
(I have seen 'Temporary building ' mean 'prefab' so perhaps 'building' could also mean 'prefab')
'porter regularly going over lovely small' is the wordplay.
'regularly going' means one should take alternating letters (letters removed at regular intervals).
'over lovely small' becomes 'fab' (I can't explain this - if you can you should believe this answer much more).
The alternate letters of 'porter' are 'pre'.
'pre'+'fab'='PREFAB'
Can you help me to learn more?
(Other definitions for prefab that I've seen before include "Factory-made building" , "Building assembled with sections made elsewhere" , "Simple construction" , "Building assembled from pre-manufactured parts, in short" , "Building assembled from prepared parts" .)