Spectator going up to support opener in the Ashes? (5)
I believe the answer is:
trees
'ashes?' is the definition.
(ash is a kind of tree)
'spectator going up to support opener in the' is the wordplay.
'spectator' becomes 'seer' (someone seeing or spectating).
'going up' says the letters should be written in reverse (in a down clue, the letters are written upwards).
'to support' means one lot of letters go next to another (in a down clue, some letters hold up others).
'opener in' suggests taking the first letters (the letter which opens the word).
The first letter of 'the' is 't'.
'seer' written backwards gives 'rees'.
'rees' put after 't' is 'TREES'.
(Other definitions for trees that I've seen before include "Elms, pines etc" , "Long lock of hair" , "Ashes, perhaps?" , "Tall, perennial, woody plant" , "Large plants" .)