On the ground, perhaps, too weak to stand up? (4)
I believe the answer is:
thin
'on the ground' is the definition.
Both the answer and definition are adverbs. Perhaps there's an association between them I don't understand?
'too weak to stand up?' is the wordplay.
'too weak' becomes 'nih' (I can't explain this - if you can you should believe this answer much more).
'stand' says to take the initial letters (I've seen 'standing' mean this).
'up?' is a reversal indicator (I've seen 'upped' mean this (in down clue, made to go up)).
The initial letter of 'to' is 't'.
'nih'+'t'='niht'
'niht' back-to-front is 'THIN'.
'perhaps' is the link.
Can you help me to learn more?
(Other definitions for thin that I've seen before include "Insubstantial" , "as 7 [LOAF] may be sliced" , "Watered down" , "Slender, slight" , "Made of light material" .)