«مَا زَيْدٌ إِلَّا شَاعِرٌ»
Zayd is nothing but a poet—that is, among the relevant qualities he is a poet rather than an alternative.
The pattern mā ... illā (ما ... إلا) opens a negated field and then exempts one attribute or individual. If the disputed properties are writing and poetry, the utterance retains poetry and excludes writing. Its force comes from denial followed by exception.
The role of the constituent after illā must be determined from syntax and the intended predication. “Mā Zaydun illā shāʿir” selects an attribute; “mā fī al-dāri illā Zayd” selects the individual who bears the attribute of presence.