Feeble border penetrated by a thousand (4)
I believe the answer is:
limp
'feeble' is the definition.
(I've seen this before)
'border penetrated by a thousand' is the wordplay.
'border' becomes 'lip' (I've seen this in other clues).
'penetrated by' indicates putting letters inside (I've seen this in other clues).
'a thousand' becomes 'm' ('M' can be a synonym of 'thousand').
'lip' placed around 'm' is 'LIMP'.
(Other definitions for limp that I've seen before include "Walk with an injury" , "Not stiff or firm" , "Walk with an uneven step" , "Walk with uneven gait" , "Walk with an uneven stop" .)