Northern girl trapping extremely active moles, perhaps (5)
I believe the answer is:
naevi
'moles perhaps' is the definition.
(plural of naevus, a type of mole on the skin)
'northern girl trapping extremely active' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'girl' becomes 'Vi' (short for Violet).
'trapping' indicates putting letters inside.
'extremely' says to hollow out the word (remove centre letters).
'active' with its centre removed is 'ae'.
'n'+'vi'='nvi'
'nvi' going around 'ae' is 'NAEVI'.
(Other definitions for naevi that I've seen before include "Spots" , "Blemishes" , "Birthmarks" , "Patches of red" , "Moles" .)