A tendency to keep son away (6)
I believe the answer is:
absent
'away' is the definition.
('absent' can be similar in meaning to 'away')
'a tendency to keep son' is the wordplay.
'tendency' becomes 'bent' (bent is a kind of tendency).
'to keep' indicates putting letters inside.
'son' becomes 's'.
'bent' going around 's' is 'bsent'.
'a'+'bsent'='ABSENT'
(Other definitions for absent that I've seen before include "Not where one should be" , "Not attending" , "Not present - vacuous" , "Missing or dreamy" , "Not at a specified place" .)