It’s weak, but we all have it (5)
I believe the answer is:
flesh
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'it's weak but we all' 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...
'have' acts as a link.
Can you help me to learn more?
(Other definitions for flesh that I've seen before include "Body, not spirit" , "Shelf (anag.)" , "Excess weight" , "Edible fruit pulp" , "See 15" .)