Frenchman turned up in India, about to become lewd (6)
I believe the answer is:
impure
'lewd' is the definition.
(I've seen this before)
'frenchman turned up in india about to' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'turned' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'india' becomes 'i' (phonetic alphabet: alpha, bravo, charlie etc.).
'about to' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'up' back-to-front is 'pu'.
'm'+'pu'='mpu'
'i'+'re'='ire'
'mpu' inserted within 'ire' is 'IMPURE'.
'become' is the link.
(Other definitions for impure that I've seen before include "Lewd" , "Sullied or adulterated" , "Adulterated or unchaste" , "Unrefined" , "Assign a value to" .)