Boris struggling to keep one in line (6)
I believe the answer is:
isobar
'line' is the definition.
(isobar is a kind of line)
'boris struggling to keep one' is the wordplay.
'struggling' indicates an anagram (the letters struggle or wriggle into a new form).
'to keep' is an insertion indicator.
'one' becomes 'a' (a thing is one thing).
'boris' with letters rearranged gives 'isobr'.
'isobr' enclosing 'a' is 'ISOBAR'.
'in' is the link.
(Other definitions for isobar that I've seen before include "sign of pressure" , "Line of equal barometric pressure on a map" , "One showing pressure" , "Map-line showing places with equal air pressure" , "Weather map feature" .)