Economise? It's serving no purpose (7)
I believe the answer is:
useless
'serving no purpose' is the definition.
(having no purpose or function)
'economise? it's' is the wordplay.
I cannot quite understand how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for useless that I've seen before include "Not worth a jot" , "Of no practical application, not worth keeping" , "Worth zilch" , "Poor" , "Fruitless" .)