Ruminant and insect run away to get married (8)
I believe the answer is:
antelope
'ruminant' is the definition.
(antelope is a kind of ruminant)
'insect run away to get married' is the wordplay.
'insect' becomes 'an't' (ant is a kind of insect).
'run away to get married' becomes 'elope' (to elope is to get married in secret).
'ant'+'elope'='ANTELOPE'
'and' acts as a link.
(Other definitions for antelope that I've seen before include "eg Eland" , "Reebok perhaps" , "Fast runner" , "A lone pet as a ruminant" , "Gazelle or impala" .)