Son called during trial — most odd (9)
I believe the answer is:
strangest
'most odd' is the definition.
(the most strange or most odd)
'son called during trial' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'called' becomes 'rang' ('ring' can be a synonym of 'call').
'during' indicates putting letters inside.
'trial' becomes 'test' (synonyms).
'rang' going into 'test' is 'trangest'.
's'+'trangest'='STRANGEST'
(Other definitions for strangest that I've seen before include "Most remarkable" , "Most unusual, oddest" , "Most unusual or odd" , "Most peculiar" , "Most out of the ordinary and unexpected" .)