Man investing in firm substance for stopping leaks (7)
I believe the answer is:
sealant
'substance for stopping leaks' is the definition.
(sealant is used to stop leaks)
'man investing in firm' is the wordplay.
'man' becomes 'alan' ().
'investing in' indicates putting letters inside.
'firm' becomes 'set' (both can mean fixed).
'alan' going into 'set' is 'SEALANT'.
(Other definitions for sealant that I've seen before include "Material for plugging leaks" , "Substance filling gap to prevent leakage" , "Bond" , "Substance for closing something tightly" , "Material used for an airtight or watertight join" .)