Strangely cruel and extremely senile hermit (7)
I believe the answer is:
recluse
'hermit' is the definition.
(synonyms)
'strangely cruel and extremely senile' is the wordplay.
'strangely' is an anagram indicator.
'and' is a charade indicator (letters next to each other).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'senile' with its middle taken out is 'se'.
'cruel' is an anagram of 'reclu'.
'reclu'+'se'='RECLUSE'
(Other definitions for recluse that I've seen before include "Miss Havisham" , "Person who lives alone and avoids people" , "One who lives alone avoiding people" , "Solitary person who avoids others" , "Person who shuns companionship, lives alone" .)