To do so legally is so legitimate! (7)
I believe the answer is:
solicit
'to do so legally' is the definition.
I can't judge whether this defines the answer.
'so legitimate' is the wordplay.
'legitimate' becomes 'licit' ('licit' can be a synonym of 'legitimate').
'so'+'licit'='SOLICIT'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for solicit that I've seen before include "Seek to obtain, ask earnestly" , "Request urgently and/or persistently" , "Make request (for business etc)" , "Ask earnestly,seek to obtain" , "Beg or court" .)