King tucks into amazingly lean joint (5)
I believe the answer is:
ankle
'joint' is the definition.
(joint of the body)
'king tucks into amazingly lean' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'tucks into' means one lot of letters goes inside another.
'amazingly' indicates an anagram.
'lean' is an anagram of 'anle'.
'k' inserted inside 'anle' is 'ANKLE'.
(Other definitions for ankle that I've seen before include "Lower leg joint" , "Location of the talus" , "Bone over foot" , "Part of the leg between the foot and the shin" , "Narrow part of leg" .)