The last supporter in army column starts being mournful (7)
I believe the answer is:
elegiac
'mournful' is the definition.
(synonym)
'the last supporter in army column starts' is the wordplay.
'last' indicates one should take the final letters.
'supporter' becomes 'leg' (I've seen this in another clue).
'starts' suggests taking the first letters.
The final letter of 'the' is 'e'.
The first letters of 'in army column' is 'iac'.
'e'+'leg'+'iac'='ELEGIAC'
'being' is the link.
(Other definitions for elegiac that I've seen before include "Funereal" , "Mournful like a sorrowful poem" , "Mournful or sad as poem may be" , "of valedictory works" , "Expressing poetic sorrow for something past" .)