Extremely reclusive mammal is close again (6)
I believe the answer is:
reseal
'is close again' is the definition.
(to seal or close again)
'extremely reclusive mammal' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'mammal' becomes 'seal' (type of mammal).
'reclusive' with its middle taken out is 're'.
're'+'seal'='RESEAL'
(Other definitions for reseal that I've seen before include "Close securely again" , "Close again tightly" .)