Sides of langoustine consumed ... off-colour in French city (5)
I believe the answer is:
lille
'french city' is the definition.
(I've seen this before)
'sides of langoustine consumed off-colour' is the wordplay.
'sides of' suggests removing the centre (only the letters on the sides remain).
'consumed' means one lot of letters goes inside another.
'off colour' becomes 'ill' (I've seen this before).
'langoustine' with its centre removed is 'le'.
'le' enclosing 'ill' is 'LILLE'.
'in' is the link.
(Other definitions for lille that I've seen before include "City in northern France, Flanders" , "University city in French Flanders" , "Industrial city of northern France" , "Feeling unwell in the French town" , "[FRANCOPHOBIA] would prevent you appreciating it" .)