One side of a coin (5)
(Other definitions for heads that I've seen before include "Coin-tossing call" , "Goes for" , "Leaders" , "Obverse result of coin-tossing" , "Ship's latrine; addicts" .)
(Other definitions for tails that I've seen before include "Follows closely" , "the back of bears - bulls too" , "Coin-tossing call" , "Ends" , "as opposed to heads" .)