I marry to secure sex and reproduce (7)
I believe the answer is:
imitate
'reproduce' is the definition.
(I know that imitate is a more specific form of reproduce)
'i marry to secure sex' is the wordplay.
'marry' becomes 'mate' (I've seen this in another clue).
'to secure' means one lot of letters goes inside another (inserted letters are secured inside).
'sex' becomes 'it' (common euphemism).
'mate' placed around 'it' is 'mitate'.
'i'+'mitate'='IMITATE'
'and' is the link.
(Other definitions for imitate that I've seen before include "I ate Tim! (anag.)" , "I tame it (anag)" , "Mock" , "Follow suit" , "Copy or mimic" .)