19 down
Less
sensible
son
gets
involved
making
little
difference
(7)
Less sensible son gets involved making little difference (7)
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The definition is at the beginning of the clue.
How the clue works
Less sensible means more insane, giving INSANER. The word INANER (a comparative meaning 'making little difference', i.e. more inane) has S (son) inserted into it, splitting it as IN-S-ANER.