Act badly, losing lead in a board game (5)
I believe the answer is:
halma
'board game' is the definition.
(halma is a kind of board game)
'act badly losing lead in a' is the wordplay.
'act badly' becomes 'ham' (as in hamming it up).
'lead' indicates taking the first letters.
'in' means one lot of letters goes inside another.
The first letter of 'losing' is 'l'.
'ham' enclosing 'l' is 'halm'.
'halm'+'a'='HALMA'
(Other definitions for halma that I've seen before include "Board-game" , "played on board" , "Leaping board game" , "Hopping board game" , "Old board game" .)