Literary genre: some commend it for imagination first of all (3-2)
I believe the answer is:
sci-fi
'literary genre' is the definition.
(I've seen this before)
'some commend it for imagination first of all' is the subsidiary indication.
'first of all' indicates taking the first letters.
The initial letters of 'some commend it for imagination' is 'scifi'.
(Other definitions for sci-fi that I've seen before include "Doctor Who genre" , "Space fiction" , "Literary genre (abbrev)" , "Sort of book" , "SF" .)