Spot, or half-spot, porter’s office (7)
I believe the answer is:
splodge
'spot' is the definition.
(I know that splodge is a type of spot)
'half-spot porter's office' is the wordplay.
'half' means to take half the letters.
'porter's office' becomes 'lodge' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'spot' halved is 'sp'.
'sp'+'lodge'='SPLODGE'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for splodge that I've seen before include "big splash" , "Irregular blot" , "Messy spot" , "Spot, splash, smear" , "Dirty mark" .)