Being old-fashioned gets a troop leader in dead trouble! (5)
I believe the answer is:
dated
'being old-fashioned' is the definition.
'a troop leader in dead trouble' is the wordplay.
'leader' says to take the initial letters.
'in' indicates putting letters inside.
'trouble' indicates an anagram.
The first letter of 'troop' is 't'.
'dead' with letters rearranged gives 'daed'.
't' placed into 'daed' is 'DATED'.
'gets' is the link.
(Other definitions for dated that I've seen before include "Pass" , "Old-fashioned" , "square" , "Went out with - old hat" , "Met socially by arrangement" .)