Thin and delicate current agent put on weight (5)
I believe the answer is:
wispy
'thin and delicate' is the definition.
(I've seen this in another clue)
'current agent put on weight' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'agent' becomes 'spy' (spy is a kind of agent).
'put on' says to put letters next to each other (some letters go on or after others).
'weight' becomes 'w' (short for weight).
'i'+'spy'='ispy'
'ispy' after 'w' is 'WISPY'.
(Other definitions for wispy that I've seen before include "Light and fine in texture" , "Lacking in distinction" , "Slender and delicate" , "Flimsy" , "Diaphanous" .)