Poor clothing that's selected for climbing (3,4)
I believe the answer is:
low gear
'selected for climbing' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'poor clothing' is the wordplay.
'poor' becomes 'low' ('low' can be similar in meaning to 'poor').
'clothing' becomes 'gear' (I've seen this before).
'low'+'gear'='LOW GEAR'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for low gear that I've seen before include "for hill-climbing?" , "Cars move slowly in it" , "It's needed on steep hill" , "It's engaged while driving up or down a steep hill" , "in which one may crawl" .)