From ACT Wiki
A potentially strong form of protection for lenders/investors in securities, designed to mitigate the adverse effects of call risk for investors.
Under a Spens clause the borrower/issuer has to value the cash flows beyond the date of the call/redemption at the government bond yield, or some other low rate.
This potentially makes it prohibitively expensive for the issuer to take an early redemption.
For example the Bank of England's purchase scheme for corporate bonds favours bonds having a Spens clause.
The consequence of a Spens clause for the investor is that they can re-invest the redemption monies in government stock, thus preserving their originally expected cash inflows at lower risk.