Ungrateful at heart. It connects foot and leg (5)
I believe the answer is:
ankle
'leg' is the definition.
(thesaurus)
'ungrateful at heart' is the wordplay.
'ungrateful' becomes 'thankless' (similar in meaning).
'at heart' says to take the centre.
The middle of 'thankless' is 'ANKLE'.
'it connects foot and' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ankle that I've seen before include "Bone over foot" , "Major foot joint" , "Foot-leg connector" , "Major joint in lower limb" , "Leg-joint" .)