Pounds invested in hollow blueprint (4)
I believe the answer is:
plan
'blueprint' is the definition.
(thesaurus)
'pounds invested in hollow' is the wordplay.
'pounds' becomes 'l' (L can represent the monetary pound).
'invested in' indicates putting letters inside.
'hollow' becomes 'pan' (I've seen this in other clues).
'l' inserted inside 'pan' is 'PLAN'.
(Other definitions for plan that I've seen before include "Detailed proposal" , "Scale drawing; scheme" , "Project" , "Method laid out for accomplishing something" , "'Scheme, strategy (4)'" .)