Ancient Goth, a bit of a liar at heart (7)
I believe the answer is:
goliath
'ancient goth a bit of a liar at heart' is the definition.
I can't tell whether this definition defines the answer.
'goth a bit of a liar at heart' is the wordplay.
'a bit of a liar' becomes 'lia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'at heart' indicates putting letters inside (inserted letters go to the heart of the word).
'goth' enclosing 'lia' is 'GOLIATH'.
Can you help me to learn more?
(Other definitions for goliath that I've seen before include "an unusually tall person" , "David slew him with a slingshot (7)" , "Philistine giant slain by David" , "Giant Philistine killed by David" , "Giant David slew" .)