I will pursue small shelf (4)
I believe the answer is:
sill
'shelf' is the definition.
(I've seen this before)
'i will pursue small' is the wordplay.
'i will' becomes 'ill' (I've seen this before).
'pursue' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ill' put after 's' is 'SILL'.
(Other definitions for sill that I've seen before include "Ledge beneath a window" , "Underwater rock ledge" , "Horizontal intrusion of igneous rock between sedimentaries, as in much of Hadrian's Wall" , "Shelf at the foot of a window" , "Horizontal timber forming the bottom of a framework" .)