Jack and king Welshman rejected (5)
I believe the answer is:
knave
'jack' is the definition.
(synonyms)
'king welshman rejected' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'welshman' becomes 'evan' (I've seen this before).
'rejected' shows that the letters should be reversed in order.
'evan' in reverse letter order is 'nave'.
'k'+'nave'='KNAVE'
'and' is the link.
(Other definitions for knave that I've seen before include "A rogue as in playing cards" , "Unscrupulous fellow" , "Unscrupulous man; jack" , "cheat of old" , "geezer" .)