Low point in Charlton or Lewisham at first (3)
I believe the answer is:
col
'low point' is the definition.
(low point between two peaks)
'charlton or lewisham at first' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'charlton or lewisham' is 'COL'.
'in' is the link.
(Other definitions for col that I've seen before include "route through mountains" , "Feature of mountain" , "Pass between hills" , "Mustard" , "Depression among ecologists" .)