Send a couple of letters by taxi (5)
I believe the answer is:
cable
'send' is the definition.
(send a message by cable)
'a couple of letters by taxi' is the wordplay.
'a couple of letters' means two replacements for 'letter'.
'letter' becomes 'l' ().
'letter' becomes 'e'.
'by' means one lot of letters go next to another.
'taxi' becomes 'cab' (synonyms).
'l'+'e'='le'
'le' after 'cab' is 'CABLE'.
(Other definitions for cable that I've seen before include "Rope for communication" , "Communications wire" , "Communication - wire - 200 yards in length" , "Strong rope made of twisted strands of hemp, nylon or steel wire" , "Anchor chain; type of TV" .)