Lofty ambitions may belie frailness at heart (4)
I believe the answer is:
tall
'lofty' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tall that I've seen before include "Towering" , "Higher than the average person" , "Of great stature" , "High in stature" , "Above average height" .)