Son lost it: any reason? (6)
I believe the answer is:
sanity
'reason?' is the definition.
(reason is a kind of sanity)
'son lost it any' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'lost' indicates an anagram.
'it'+'any'='itany'
'itany' anagrammed gives 'anity'.
's'+'anity'='SANITY'
(Other definitions for sanity that I've seen before include "Rational behaviour" , "Balance of mind" , "Rational condition" , "Soundness of mind" , "presence of mind?" .)