Planet Hollywood's opening in Slough (5)
I believe the answer is:
marsh
'slough' is the definition.
(both can mean a swampy area)
'planet hollywood's opening' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'opening' indicates taking the first letters.
The initial letter of 'hollywoods' is 'h'.
'mars'+'h'='MARSH'
'in' acts as a link.
(Other definitions for marsh that I've seen before include "eg Romney" , "Fen" , "Low-lying wet land with grassy vegetation" , "Bog, swamp" , "Wet land; swamp" .)