Urge one to join Liberal Party (6)
I believe the answer is:
libido
'urge' is the definition.
(I've seen this before)
'one to join liberal party' is the wordplay.
'one to join' means one lot of letters go next to another.
'liberal' becomes 'lib'.
'party' becomes 'ido' (I can't explain this - if you can you should believe this answer much more).
'lib'+'ido' is 'LIBIDO'.
Can you help me to learn more?
(Other definitions for libido that I've seen before include "Freud's sex drive" , "Instinctive desire" , "Life force" , "Vital urge" , "Emotional craving" .)