Prohibit going either way in crossing (6)
I believe the answer is:
forbid
'prohibit' is the definition.
(I know that prohibit can be written as forbid)
'going either way in crossing' is the wordplay.
'going either way' becomes 'bi' (bisexual).
'in' is an insertion indicator.
'crossing' becomes 'ford' (ford is a kind of crossing).
'bi' placed inside 'ford' is 'FORBID'.
(Other definitions for forbid that I've seen before include "Refuse permission, order not to" , "Outlaw" , "Ban" , "Refuse permission, do not allow" , "Deny, prohibit" .)