Pip imported aluminium. Watertight (6)
I believe the answer is:
sealed
'watertight' is the definition.
(thesaurus)
'pip imported aluminium' is the wordplay.
'pip' becomes 'seed' (I've seen this before).
'imported' is an insertion indicator.
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'seed' enclosing 'al' is 'SEALED'.
(Other definitions for sealed that I've seen before include "Closed (of lips, envelope, tomb etc)" , "Closed up, watertight" , "Closed tightly, maybe with wax" , "Airtight" , "Stamped as authentic" .)