Mother on trial, getting out of marriage (7)
I believe the answer is:
marital
'of marriage' is the definition.
(I've seen this before)
'mother on trial getting out' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'on' means one lot of letters go next to another.
'getting out' is an anagram indicator (the letters get out of order).
'trial' anagrammed gives 'rital'.
'ma'+'rital'='MARITAL'
(Other definitions for marital that I've seen before include "to do with life together" , "Sort of union" , "Connubial" , "Relating to union of husband and wife" , "Of a wedding" .)