Son telephoned during exam -- most unusual! (9)
I believe the answer is:
strangest
'most unusual' is the definition.
(I've seen this before)
'son telephoned during exam' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'telephoned' becomes 'rang' ('ring' can be a synonym of 'telephone').
'during' means one lot of letters goes inside another.
'exam' becomes 'test' (synonyms).
'rang' inserted into 'test' is 'trangest'.
's'+'trangest'='STRANGEST'
(Other definitions for strangest that I've seen before include "Most peculiar" , "Most out of the ordinary and unexpected" , "Most unusual or odd" , "Most unusual, oddest" , "Most remarkable" .)