Love and marriage, don't start that as it can make you cry! (5)
I believe the answer is:
onion
'it can make you cry' is the definition.
'love and marriage don't start that' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'and' says to put letters next to each other.
'marriage' becomes 'union' ('union' can be a synonym of 'marriage').
'don't start that' means to remove the first letter.
'union' with its initial letter removed is 'nion'.
'o'+'nion'='ONION'
'as' acts as a link.
(Other definitions for onion that I've seen before include "One may be set" , "it can bring tears" , "one understood by expert" , "Shape of Russian dome" , "The tear-inducing vegetable" .)