Gross behind? It's fantastic! Instinct's to visit (her husband's often away) (5,5)
I believe the answer is:
grass widow
'visit her husband's often away' is the definition.
I can't judge whether this defines the answer.
'gross behind? it's fantastic instinct's' is the wordplay.
'gross' becomes 'gr' (abbreviation).
'behind?' becomes 'ass' (both can mean one's back side).
'it's fantastic instinct' becomes 'widow' (I can't justify this - if you can you should believe this answer much more).
'gr'+'ass'+'widow'='GRASS-WIDOW'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for grass widow that I've seen before include "Her husband's often away -" , "Lonely woman" , "Sportsman's wife" , "Being often left alone by husband" , "Spouse of absent husband" .)