Line on map runs into field (6)
I believe the answer is:
tropic
'line on map' is the definition.
'runs into field' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' is an insertion indicator.
'field' becomes 'topic'.
'r' placed within 'topic' is 'TROPIC'.
(Other definitions for tropic that I've seen before include "Cancer is one, Capricorn another" , "One of two parallels of latitude" , "One of two parallels, Cancer or Capricorn" , "Capricorn, say" , "Either that of Capricorn or Cancer" .)