That one, on the outside, looks quite masculine, by and large. (6)
I believe the answer is:
mainly
'by and large' is the definition.
(I've seen this in another clue)
'that one on the outside looks quite masculine' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'masculine' could be 'manly' (similar in meaning) and 'manly' is located in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mainly that I've seen before include "Generally or largely" , "For the most part" , "Chiefly, largely" , "Chiefly, principally" .)