One game troop leader finds unsuitable! (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(thesaurus)
'one game troop leader' is the wordplay.
'one' becomes 'i' (Roman numeral).
'game' becomes 'nap' (I've seen this before**).
'leader' says to take the initial letters.
The first letter of 'troop' is 't'.
'i'+'nap'+'t'='INAPT'
'finds' is the link.
(Other definitions for inapt that I've seen before include "not fit for purpose" , "Not proper" , "Unsuitable - paint (anag)" , "Useless" , "not seemly" .)