Inside shop a queue's becoming dense (6)
I believe the answer is:
opaque
'dense' is the definition.
(thesaurus)
'inside shop a queue's' is the wordplay.
'inside' says the answer is hidden in the clue.
'OPAQUE' can be found hidden inside 'shop a queues'.
'becoming' is the link.
(Other definitions for opaque that I've seen before include "Cloudy, not transparent" , "Not transparent, light can't pass through" , "Cloudy, non-transparent" , "Impenetrable to light" , "Not translucent" .)