Criminal group extremely lanky: tall and thin (6)
I believe the answer is:
gangly
'tall and thin' is the definition.
(gangly means lanky)
'criminal group extremely lanky' is the wordplay.
'criminal group' becomes 'gang' (a criminal gang).
'extremely' suggests removing the centre (only the extremes of the word are used).
'lanky' with its centre taken out is 'ly'.
'gang'+'ly'='GANGLY'
(Other definitions for gangly that I've seen before include "Tall and spindly" , "Loosely-built, lanky" , "With long, awkward limbs" , "Tall, thin, awkward" , "Tall and awkward in movement" .)