Lad has invested almost everything in hairdressing shop (5)
'hairdressing shop' is the definition.
(a hair salon)
'lad has invested almost everything' is the wordplay.
'lad' becomes 'son' ().
'has invested' is an insertion indicator.
'almost' means to remove the last letter (most of the word but not all of it).
'everything' becomes 'all' (I've seen this before).
'all' with its final letter taken off is 'al'.
'son' enclosing 'al' is 'SALON'.
'in' acts as a link.
(Other definitions for salon that I've seen before include "Formal sitting room", "Hairdresser's premises", "Loans out as a reception room", "social event", "Gallery for works of art -- loans (anag)".)