Maybe fighter’s fitness trainer who is unusually small (13)
I believe the answer is:
airworthiness
'maybe fighter's fitness' is the definition.
(eg a fighter jet's fitness for flying)
'trainer who is unusually small' is the wordplay.
'unusually' indicates an anagram.
'small' becomes 's' (abbreviation - e.g. clothes size).
'trainer'+'who'+'is'='trainerwhois'
'trainerwhois' with letters rearranged gives 'airworthines'.
'airworthines'+'s'='AIRWORTHINESS'
(Other definitions for airworthiness that I've seen before include "Condition for taking off" , "requirement for flight" , "Capability to fly (of planes)" , "high flyers need it" , "flying certificate?" .)