Dad chopping tree up for timber (5)
I believe the answer is:
maple
'timber' is the definition.
(maple is a type of timber)
'dad chopping tree up' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'chopping' means one lot of letters goes inside another (inserted letters chop into others).
'tree' becomes 'elm' (elm tree is a kind of tree).
'up' is a reversal indicator (in down clue: letters go upwards).
'pa' going within 'elm' is 'elpam'.
'elpam' back-to-front is 'MAPLE'.
'for' is the link.
(Other definitions for maple that I've seen before include "Canadian flag leaf" , "Tree providing syrup" , "Tree yielding syrup" , "Emblematic tree of Canada" , "Cathedral city in Devon" .)