Item of tack or lumber (6)
I believe the answer is:
saddle
This clue could be a double definition.
'item of tack' is the first definition.
'saddle' can be an answer for 'tack' (thesaurus). I am not sure about the 'item of' bit.
'lumber' is the second definition.
(I've seen this before)
'or' acts as a link.
Can you help me to learn more?
(Other definitions for saddle that I've seen before include "Cycle seat" , "Impose a task on -- assign responsibility for" , "load" , "Cut of meat" , "Burden: pass" .)