A good measure introduced by fellow who went on long expedition? (8)
I believe the answer is:
magellan
'who went on long expedition?' is the definition.
(explorer Ferdinand Magellan)
'a good measure introduced by fellow' is the wordplay.
'good' becomes 'g' (abbreviation).
'measure' becomes 'ell' (historical unit of length).
'introduced by' is an insertion indicator (I've seen this in other clues).
'fellow' becomes 'man' (I've seen this before).
'a'+'g'+'ell'='agell'
'agell' put inside 'man' is 'MAGELLAN'.
(Other definitions for magellan that I've seen before include "Port. explorer; a S. Am. Strait" , "Portuguese navigator, killed 1521" , "Major strait between the Atlantic and Pacific" , "S American strait named for 16th century Portuguese explorer" , "Port. navigator" .)