Letter finds most of them joining Territorial Army (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter)
'most of them joining territorial army' is the wordplay.
'most of' means to remove the last letter.
'joining' says to put letters next to each other.
'territorial army' becomes 'ta'.
'them' with its final letter removed is 'the'.
'the'+'ta'='THETA'
'finds' is the link.
(Other definitions for theta that I've seen before include "classic character?" , "character in Marathon" , "letter from abroad" , "Make amends for" , "Foreign letter" .)