Harriet's underwear enthralled long-legged individual (10)
I believe the answer is:
harvestman
'long-legged individual' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'harriet's underwear enthralled' is the wordplay.
'harriet's' becomes 'harman' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'underwear' becomes 'vest'.
'enthralled' means one lot of letters goes inside another.
'harman' placed around 'vest' is 'HARVESTMAN'.
Can you help me to learn more?
(Other definitions for harvestman that I've seen before include "It's an arachnid" , "one gathers" .)