Without limits, invest in pound (3)
I believe the answer is:
pen
'pound' is the definition.
(I've seen this in other clues)
'without limits invest in' is the wordplay.
'without limits' means to look at the middle letters (letters at the edges or 'limits' are removed).
'invest in' becomes 'spend'.
The central letters of 'spend' are 'PEN'.
(Other definitions for pen that I've seen before include "Writing implement" , "International Association of poets, playwrights etc" , "Cage - bird" , "Pound" , "Enclosure; a swan" .)