Ambassador’s absorbed by sign one will offer alternative terms (9)
I believe the answer is:
thesaurus
'one will offer alternative terms' is the definition.
I can't tell whether this defines the answer.
'ambassador's absorbed by sign' is the wordplay.
'ambassador's' becomes 'HE's' (His or Her Excellency).
'absorbed by' means one lot of letters goes inside another.
'sign' becomes 'taurus' (Taurus is an example).
'hes' going within 'taurus' is 'THESAURUS'.
Can you help me to learn more?
(Other definitions for thesaurus that I've seen before include "Wordy gathering" , "Volume of synonyms" , "Reference work" , "Book of words with synonyms, antonyms etc" , "storehouse" .)