Great folly girl's built in northern area (6)
I believe the answer is:
lunacy
'great folly' is the definition.
'lunacy' can be an answer for 'folly' (I've seen this before). I am not certain of the 'great' bit.
'girl's built in northern area' is the wordplay.
I cannot quite see how this works, but
'northern' could be 'n' (abbreviation e.g. N. Ireland) and 'n' is present in the answer.
'area' could be 'a' (maths abbreviation) and 'a' is found in the answer.
'girl' could be 'lu' and 'lu' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lunacy that I've seen before include "Extreme folly, insanity" , "Tomfoolery" , "idiotic act" , "Craziness, madness" , "Moon madness" .)