Rod has to understand that woman to remain married? (5,8)
I believe the answer is:
stick together
'to remain married?' is the definition.
'rod has to understand that woman' is the wordplay.
'rod' becomes 'stick' (I've seen this before).
'has' means one lot of letters go next to another.
'understand' becomes 'get' (getting is a kind of understanding).
'that woman' becomes 'her' (I've seen this before**).
'stick'+'to'+'get'+'her'='STICK TOGETHER'
(Other definitions for stick together that I've seen before include "maintain united front" , "Remain united" .)