Buck up everyone at the front and behind (5)
I believe the answer is:
elate
'buck up' is the definition.
(I've seen this in another clue)
'everyone at the front and behind' is the wordplay.
'at the front' indicates taking the first letters.
'and' means one lot of letters go next to another.
'behind' becomes 'late' (as in behind schedule).
The initial letter of 'everyone' is 'e'.
'e'+'late'='ELATE'
(Other definitions for elate that I've seen before include "Fill with optimism" , "Lift (the spirits)" , "Inspirit, make exultant" , "Exhilarate, put in high spirits" , "Raise the spirits of" .)