Shy sub, half demented (7)
I believe the answer is:
bashful
'shy' is the definition.
(thesaurus)
'sub half demented' is the wordplay.
'demented' indicates anagramming the letters.
'sub'+'half'='subhalf'
'subhalf' anagrammed gives 'BASHFUL'.
(Other definitions for bashful that I've seen before include "reserved" , "Neither Happy, Sneezy, Dopey, Grumpy, Sleepy nor Doc!" , "Sleepy companion" , "Shy and self-conscious" , "Shovel and pick worker" .)