It slithers around (in summer?) (5)
I believe the answer is:
adder
'summer?' is the definition.
(someone who adds or does sums)
'it slithers around' is the wordplay.
I cannot quite see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
The remaining letters 'addr' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for adder that I've seen before include "Mathematical reptile" , "Snake (like an abacus?)" , "One contributing more" , "accountant?" , "poisoner" .)