TV science programme leading discussion mostly on the level (10)
I believe the answer is:
horizontal
'the level' is the definition.
(thesaurus)
'tv science programme leading discussion mostly' is the wordplay.
'tv science programme' becomes 'Horizon' (BBC science programme).
'leading' means one lot of letters go next to another.
'discussion' becomes 'talk' (talk is a kind of discussion).
'mostly' means to remove the last letter.
'talk' with its last letter taken off is 'tal'.
'horizon'+'tal'='HORIZONTAL'
'on' acts as a link.
(Other definitions for horizontal that I've seen before include "Lying flat" , "Prone" , "even" , "What this answer isn't!" .)