Ward off fighter, an undersized one (5)
I believe the answer is:
dwarf
'an undersized one' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'ward off fighter' is the wordplay.
'off' is an anagram indicator.
'fighter' becomes 'F' (abbreviation in war plane e.g. F-16 etc.).
'ward' is an anagram of 'dwar'.
'dwar'+'f'='DWARF'
Can you help me to learn more?
(Other definitions for dwarf that I've seen before include "Friend of Snow White?" , "Tower over" , "Small person or star" , "Little red sky feature" , "Metal-worker of legendary tales" .)