Limits phosphorus in agricultural device for small bird (5,7)
I believe the answer is:
hedge sparrow
'for small bird' is the definition.
(hedge sparrows are small birds)
'limits phosphorus in agricultural device' is the wordplay.
'limits' becomes 'edges' (I have seen 'Outside limits ' mean 'edges' so perhaps 'limits' could also mean 'edges').
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'in' indicates putting letters inside.
'agricultural device' becomes 'harrow' (I've seen this before).
'edges'+'p'='edgesp'
'edgesp' going inside 'harrow' is 'HEDGE-SPARROW'.
(Other definitions for hedge sparrow that I've seen before include "Dunnock" , "Small brownish songbird" , "Bird" , "Small European songbird" .)