I love mostly pleasant; architectural style (5)
I believe the answer is:
ionic
'architectural style' is the definition.
(I've seen this in another clue)
'i love mostly pleasant' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'mostly' means to remove the last letter (most but not all of the word).
'pleasant' becomes 'nice' (similar in meaning).
'nice' with its last letter removed is 'nic'.
'i'+'o'+'nic'='IONIC'
(Other definitions for ionic that I've seen before include "Description of bond" , "Classical style" , "classical Greek design" , "Classic form of dialect" , "in ancient Greece?" .)