Mother, mostly pleasant, is very busy (5)
I believe the answer is:
manic
'very busy' is the definition.
'mother mostly pleasant' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'mostly' means to remove the last letter.
'pleasant' becomes 'nice' ('nice' is associated in meaning with 'pleasant').
'nice' with its last letter removed is 'nic'.
'ma'+'nic'='MANIC'
'is' is the link.
(Other definitions for manic that I've seen before include "Insane" , "demented" , "Frenzied" , "Showing wild energy" , "crazy" .)