Undertakes large welcome for one needing work (4,5)
I believe the answer is:
wage slave
'one needing work' is the definition.
The definition suggests a singular noun which matches the answer.
'undertakes large welcome' is the wordplay.
'undertakes' becomes 'wages' (I am not sure about this - if you are sure you should believe this answer much more).
'large' becomes 'L' (eg in clothes sizes).
'welcome' becomes 'ave' ('welcome' or 'farewell' - from Latin).
'wages'+'l'+'ave'='WAGE SLAVE'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for wage slave that I've seen before include "One dependant on a salary" , "Someone compelled to work in order to survive" , "Worker" , "Person dependent on pay packet" , "Person compelled to work for pay in order to survive" .)