It’s fat and starts high and finishes low (6)
I believe the answer is:
tallow
'it's fat' is the definition.
(thesaurus)
'high and finishes low' is the wordplay.
'high' becomes 'tall' ('tall' is associated in meaning with 'high').
'and' says to put letters next to each other.
'finishes low' becomes 'ow' (I can't justify this - if you can you should give a lot more credence to this answer).
'tall'+'ow'='TALLOW'
'and starts' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tallow that I've seen before include "Animal fat used in candles, soap etc" , "Fatty animal substance in soap making" , "Rendered fat used for making candles" , "Fat processed for making candles" , "Candle wax" .)