Back being serious (5)
I believe the answer is:
stern
I believe this clue is a double definition.
'back' is the first definition.
(stern is a kind of back)
'serious' is the second definition.
(I've seen this before)
'being' is the link.
(Other definitions for stern that I've seen before include "Authoritarian, back" , "Looking severe" , "Strict - forbidding" , "Strict; severe" , "Rear end" .)