Lack the skill to hold net up. It's not going to work! (9)
I believe the answer is:
untenable
'not going to work' is the definition.
I can't judge whether this definition defines the answer.
'lack the skill to hold net up' is the wordplay.
'lack the skill' becomes 'unable' (I can't explain this - if you can you should believe this answer much more).
'to hold' means one lot of letters goes inside another.
'up' is a reversal indicator (in down clue: letters go upwards).
'net' reversed gives 'ten'.
'unable' placed around 'ten' is 'UNTENABLE'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for untenable that I've seen before include "impossible to support" , "Indefensible" , "unworthy of support?" , "Incapable of being defended or justified" , "Not defensible" .)