Being old-fashioned gets a troop leader in dead trouble! (5)
I believe the answer is:
dated
'being old-fashioned' is the definition.
(dated can mean old-fashioned)
'a troop leader in dead trouble' is the wordplay.
'leader' suggests taking the first letters.
'in' indicates putting letters inside.
'trouble' indicates an anagram.
The first letter of 'troop' is 't'.
'dead' anagrammed gives 'daed'.
't' placed inside '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" .)