Weatherman's line: 'I weep over a drop of rain' (6)
I believe the answer is:
isobar
'weatherman's line' is the definition.
(I've seen this in another clue)
'i weep over a drop of rain' is the wordplay.
'weep' becomes 'sob' (sobbing is a kind of weeping).
'over' says to put letters next to each other.
'drop of' indicates taking the first letters.
The initial letter of 'rain' is 'r'.
'i'+'sob'+'a'+'r'='ISOBAR'
(Other definitions for isobar that I've seen before include "Line that connects places of equal air pressure" , "Air pressure line on a weather chart" , "Equal-pressure line" , "Line in front" , "Line showing points having equal barometric pressure at a given time" .)