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' is an anagram indicator.
'terse' is an anagram of '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" .)