Maintained king has strange pet (4)
I believe the answer is:
kept
'maintained' is the definition.
('keep' can be a synonym of 'maintain')
'king has strange pet' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'has' says to put letters next to each other.
'strange' indicates anagramming the letters.
'pet' is an anagram of 'ept'.
'k'+'ept'='KEPT'
(Other definitions for kept that I've seen before include "Maintained, detained" , "Saved" , "Looked after" , "Retained, held back" , "Retained possession" .)