Line in answer penned by Boris, strangely (6)
I believe the answer is:
isobar
'line' is the definition.
(isobar is a kind of line)
'answer penned by boris strangely' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'penned by' means one lot of letters goes inside another.
'strangely' indicates anagramming the letters.
'boris' anagrammed gives 'isobr'.
'a' going inside 'isobr' is 'ISOBAR'.
'in' acts as a link.
(Other definitions for isobar that I've seen before include "Line on map connecting places of equal atmospheric pressure" , "Met map pressure line" , "Weather-map line" , "What's shown on some maps" , "It appears on some maps" .)