Drastically prune extremely small tree (5)
I believe the answer is:
slash
'drastically prune' is the definition.
(I've seen this before)
'extremely small tree' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'tree' becomes 'ash' (ash is a kind of tree).
'small' with its centre removed is 'sl'.
'sl'+'ash'='SLASH'
(Other definitions for slash that I've seen before include "Violently cut" , "Reduce (a price) greatly" , "Drastically prune" , "Punctuation mark" , "Lacerate" .)