Terse about oaks, say (5)
I believe the answer is:
trees
'oaks say' is the definition.
(oak is a kind of tree)
'terse about' is the wordplay.
'about' indicates an anagram.
'terse' anagrammed gives 'TREES'.
(Other definitions for trees that I've seen before include "Long lock of hair" , "Tall, perennial, woody plants" , "Steer into the plants" , "Woods" , "Elms, pines etc" .)