It has a gentle wriggle (6)
I believe the answer is:
maggot
'gentle wriggle' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'it has a' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'a' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for maggot that I've seen before include "Lava of a fly" , "youngster to fly" , "pest" , "Legless grub" , "immature flyer" .)