High (ouch!) in fat (6)
I believe the answer is:
tallow
'fat' is the definition.
(thesaurus)
'high ouch' is the wordplay.
'high' becomes 'tall' ('tall' is associated in meaning with 'high').
'ouch' becomes 'ow' (interjection synonym).
'tall'+'ow'='TALLOW'
'in' acts as a link.
(Other definitions for tallow that I've seen before include "burning fat" , "Animal oil used in making soap and candles" , "Animal fat used in making soap" , "Fatty substance used in candles, soap etc" , "Candle or soap substance" .)