It's a baffling thing to do (7)
I believe the answer is:
perplex
'do' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'it's a baffling thing' is the wordplay.
I cannot quite see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for perplex that I've seen before include "Mystify" , "Bamboozle" , "Dumbfound" , "Make (someone) baffled" , "Bewilder" .)