Man wants a kiss first as a rule (5)
I believe the answer is:
axiom
'a rule' is the definition.
(I've seen this before)
'man wants a kiss first' is the wordplay.
'man wants' becomes 'iom' (I can't explain this - if you can you should give a lot more credence to this answer).
'kiss' becomes 'x' (X can represent a kiss).
'first' means one lot of letters go next to another.
'a'+'x'='ax'
'iom' put after 'ax' is 'AXIOM'.
'as' is the link.
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "Plainly true proposition" , "Truth assumed to be self-evident" , "Clearly true statement" , "A saying that's widely accepted as true" , "saw" .)