Mother cracked trial of marriage (7)
I believe the answer is:
marital
'of marriage' is the definition.
(I've seen this before)
'mother cracked trial' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'cracked' is an anagram indicator.
'trial' with letters rearranged gives 'rital'.
'ma'+'rital'='MARITAL'
(Other definitions for marital that I've seen before include "Sort of union" , "Relating to marriage" , "Conjugal" , "This type of bliss" , "related to union" .)