Thinner internet up for development (10)
I believe the answer is:
turpentine
'thinner' is the definition.
(turpentine is used to thin paints)
'internet up for development' is the wordplay.
'for development' indicates anagramming the letters.
'internet'+'up'='internetup'
'internetup' is an anagram of 'TURPENTINE'.
(Other definitions for turpentine that I've seen before include "Oil used in varnishes" , "Thick, oily resin used as paint thinner" , "Paint/varnish spirit" , "artist's stuff" , "Solvent" .)