Whole-hearted boat-building is forbidden! (5)
I believe the answer is:
taboo
'is forbidden' is the definition.
('taboo' can be a synonym of 'forbidden')
'whole-hearted boat-building' is the wordplay.
'hearted' means to look at the middle letters.
'building' indicates an anagram.
The middle letter of 'whole' is 'o'.
'o'+'boat'='oboat'
'oboat' is an anagram of 'TABOO'.
(Other definitions for taboo that I've seen before include "Ritual prohibition" , "Anathema" , "Not permitted" , "Socially forbidden" , "(Irrational) prohibition" .)