Spill wild peat over top of rockery (5)
I believe the answer is:
taper
'spill' is the definition.
(thesaurus)
'wild peat over top of rockery' is the wordplay.
'wild' indicates an anagram.
'over' says to put letters next to each other.
'top of' suggests taking the first letters.
The initial letter of 'rockery' is 'r'.
'peat' anagrammed gives 'tape'.
'tape'+'r'='TAPER'
(Other definitions for taper that I've seen before include "Convex shape that narrows toward a point" , "Recorder" , "Peter out" , "Waxed spill" , "Get more narrow" .)