A length, therefore, might be further (4)
I believe the answer is:
also
'further' is the definition.
(thesaurus)
'a length therefore' is the wordplay.
'length' becomes 'l'.
'therefore' becomes 'so'.
'a'+'l'+'so'='ALSO'
'might be' acts as a link.
(Other definitions for also that I've seen before include "As well, in addition" , "admitted to Royal Society > too" , "Furthermore" , "into the bargain" , "What's more" .)