Project involving son only (4)
I believe the answer is:
just
'only' is the definition.
('just' can be a synonym of 'only')
'project involving son' is the wordplay.
'project' becomes 'jut' (I've seen this before**).
'involving' indicates putting letters inside.
'son' becomes 's'.
'jut' placed around 's' is 'JUST'.
(Other definitions for just that I've seen before include "Simply - fair" , "This very minute" , "By a whisker" , "Exactly" , "Barely honest?" .)