Weak resistance in tank... (5)
I believe the answer is:
frail
'weak' is the definition.
(I know that weak can be written as frail)
'resistance in tank' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'in' means one lot of letters goes inside another.
'tank' becomes 'fail' (I've seen this in another clue).
'r' going into 'fail' is 'FRAIL'.
(Other definitions for frail that I've seen before include "Decrepit" , "rush" , "Physically feeble" , "Easily shattered" , "hobbling around?" .)