Line of latitude runs into field (6)
I believe the answer is:
tropic
'line of latitude' is the definition.
(tropic is a kind of line of latitude)
'runs into field' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'field' becomes 'topic'.
'r' going into 'topic' is 'TROPIC'.
(Other definitions for tropic that I've seen before include "One of two parallels, Cancer or Capricorn" , "Latitude associated with Capricorn or Cancer" , "Interzonal line" , "Cancer is one, Capricorn another" , "Duck" .)