In a zoo you may see one close up (4)
I believe the answer is:
seal
'one close up' is the definition.
(I've seen this before)
'in a zoo you' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
The remaining letters 'sel' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'may see' acts as a link.
Can you help me to learn more?
(Other definitions for seal that I've seen before include "Secure; aquatic animal" , "A mammal; one that secures" , "Authorise - plug" , "Sea-dweller can produce a wax impression" , "Stick down" .)