Space novel is military leader’s fantasy (8)
I believe the answer is:
escapism
'fantasy' is the definition.
(I've seen this before)
'space novel is military leader's' is the wordplay.
'novel' indicates anagramming the letters.
'leader' suggests taking the first letters.
The first letter of 'military' is 'm'.
'space' is an anagram of 'escap'.
'escap'+'is'+'m'='ESCAPISM'
(Other definitions for escapism that I've seen before include "Avoiding the actual" , "The avoidance of reality by absorbing the mind in fantasy" , "Mass epic (anag.)" , "Tendency to flee reality" , "Avoiding reality by fantasising" .)