Philosopher astride horse or kind of lamb (7)
I believe the answer is:
paschal
'kind of lamb' is the definition.
(as in a paschal lamb, lamb sacrificed at Easter)
'philosopher astride horse' is the wordplay.
'philosopher' becomes 'pascal' (Pascal is an example).
'astride' means one lot of letters goes inside another.
'horse' becomes 'H' (abbreviation as in horse power).
'pascal' enclosing 'h' is 'PASCHAL'.
'or' is the link.
(Other definitions for paschal that I've seen before include "Relating to Easter" , "religious festival" .)