Transparent film’s temperature in turbulent Lake Constance (7,4)
I believe the answer is:
contact lens
'transparent film's' is the definition.
(contact lenses are transparent bits of film)
'temperature in turbulent lake constance' is the wordplay.
'temperature' becomes 't' (abbreviation).
'in' means one lot of letters goes inside another.
'turbulent' is an anagram indicator.
'lake' becomes 'L' (geographical abbreviation).
'l'+'constance'='lconstance'
'lconstance' with letters rearranged gives 'conactlens'.
't' going inside 'conactlens' is 'CONTACT LENS'.
(Other definitions for contact lens that I've seen before include "you're supposed to see through it" , "Vision corrector" , "Eyesight corrector" , "Aid to vision" , "Visual aid" .)