Moles, extremely active and very visible in Northern Ireland (5)
I believe the answer is:
naevi
'moles' is the definition.
(I've seen this before)
'extremely active and very visible in northern ireland' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'and' means one lot of letters go next to another.
'very' becomes 'v' (abbreviation).
'visible in' is an insertion indicator.
'northern ireland' becomes 'ni'.
'active' with its centre taken out is 'ae'.
'ae'+'v'='aev'
'aev' going within 'ni' is 'NAEVI'.
(Other definitions for naevi that I've seen before include "Blemishes" , "Birthmarks" , "Patches of red" , "Moles" , "Spots" .)