Sheer and occasionally delicate part of a car (9)
I believe the answer is:
headlight
'part of a car' is the definition.
(headlights are part of a car)
'sheer and occasionally delicate' is the wordplay.
'occasionally' means one should take alternating letters.
'delicate' becomes 'light' (similar in meaning).
'sheer'+'and'='sheerand'
The alternate letters of 'sheerand' are 'head'.
'head'+'light'='HEADLIGHT'
(Other definitions for headlight that I've seen before include "Powerful car lamp" , "What motorist may use" , "Car beam" , "Given shiner" , "clarifies the way forward" .)