Sign seen outside explosive's storehouse (9)
I believe the answer is:
thesaurus
'storehouse' is the definition.
(I've seen this before)
'sign seen outside explosive's' is the wordplay.
'sign' becomes 'taurus' (Taurus is an example).
'seen outside' indicates putting letters inside.
'explosive's' becomes 'he's' (abbreviation for high explosive).
'taurus' placed around 'hes' is 'THESAURUS'.
(Other definitions for thesaurus that I've seen before include "Dictionary" , "Wordy book" , "Book listing synonyms" , "work we may refer to" , "Volume of synonyms" .)