Worried about horse taking on weight (6)
I believe the answer is:
stewed
'worried' is the definition.
(as in stewing in your fears)
'horse taking on weight' is the wordplay.
'horse' becomes 'steed' (steed is a kind of horse).
'taking on' means one lot of letters goes inside another.
'weight' becomes 'w' (short for weight).
'steed' going around 'w' is 'STEWED'.
'about' acts as a link.
(Other definitions for stewed that I've seen before include "mortal" , "Of tea, left brewing too long" , "canned" , "(Meat) simmered; (tea) over-infused" , "was in agitated state" .)