Foreign leader isn't weak (5)
I believe the answer is:
faint
'weak' is the definition.
(similar in meaning)
'foreign leader isn't' is the wordplay.
'leader' indicates taking the first letters.
'isn't' becomes 'aint' (non-standard version of 'isn't').
The initial letter of 'foreign' is 'f'.
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "Indistinct - pass out" , "Pale - pass out" , "Swoon - pale" , "Weak" , "Hard to see" .)