A talent king is able to knight at first in retirement (5)
I believe the answer is:
knack
'a talent' is the definition.
(as in having the knack for something)
'king is able to knight at first in retirement' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'is able to' becomes 'can' (if you can do something you are able to do it).
'at first' indicates taking the first letters.
'in retirement' says the letters should be written in reverse (retire can mean to go back or backwards).
The first letter of 'knight' is 'k'.
'k'+'can'+'k'='kcank'
'kcank' backwards is 'KNACK'.
(Other definitions for knack that I've seen before include "Technique" , "Special ability" , "Skilful competence" , "Skill for specific task" , "Gift" .)