It's thoroughly lawful to seek their custom (7)
I believe the answer is:
solicit
'seek their custom' is the definition.
I can't tell whether this defines the answer.
'it's thoroughly lawful' is the wordplay.
'it's thoroughly' becomes 'so' (I can't explain this - if you can you should give a lot more credence to this answer).
'lawful' becomes 'licit' (synonyms).
'so'+'licit'='SOLICIT'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for solicit that I've seen before include "Petition - importune" , "Ask (someone) for something" , "Proposition" , "Accost - request" , "Drum up business" .)