Two reasons:
(i) CASHTENT (or CASH-TENT) isn't a word
(ii) If the definition were 'Part of campsite where change may be had', the word 'for' would then have to be a link FROM the definition TO the word play (Spooner's capital), which doesn't work.
As it is, the word play is 'Part of campsite where change may be had for Spooner', the link is 'is' ['s] and the definition is 'capital'.