Old woman failed trial of marriage (7)
I believe the answer is:
marital
'of marriage' is the definition.
(I've seen this before)
'old woman failed trial' is the wordplay.
'old woman' becomes 'ma' (both can mean someone's mother).
'failed' indicates anagramming the letters.
'trial' anagrammed gives 'rital'.
'ma'+'rital'='MARITAL'
(Other definitions for marital that I've seen before include "Conjugal" , "to do with couple's relationship" , "Relating to wedded life" , "Relating to union of husband and wife" , "Connubial" .)