Weak King restricted by collapse (5)
I believe the answer is:
frail
'weak' is the definition.
(synonyms)
'king restricted by collapse' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'restricted by' means one lot of letters goes inside another (I've seen this in other clues).
'collapse' becomes 'fail' (I've seen this before).
'r' going within 'fail' is 'FRAIL'.
(Other definitions for frail that I've seen before include "Physically weak, delicate" , "Struggling to get about" , "Feeble - basket" , "rush" , "Decrepit" .)