Length in lead is expressed in a foreign language (5)
I believe the answer is:
lingo
'a foreign language' is the definition.
(I've seen this before)
'length in lead' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'lead' becomes 'go' (synonyms).
'l'+'in'+'go'='LINGO'
'is expressed in' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lingo that I've seen before include "Patois" , "Foreign language or dialect: jargon" , "Johnny Foreigner speaks one" , "Log in (anag.)" , "Jargon, foreign language (informal)" .)