The sort of legs Biggles might have had? (3)
I believe the answer is:
big
'had?' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives sometimes define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for big that I've seen before include "Of considerable size" , "Circus tent" , "Massive" , "Large; generous" , "Large or substantial" .)