Probably as suggested in Redmonds Dictionary of Yorks Surnames, a variant of Spendley in which the consonant cluster /ndl/ has become pronounced /nsl/. Spendley is itself a variant of Spendlow alias Spendlove.
Redmonds provides circumstantial evidence from York Guild Register where Thomas Spensley (1477) can be compared with Walter Spenlowe (1433) and Elizabeth Spendlay (1522).
Further evidence appears in IGI data for Oswaldkirk (NR Yorks) where Johanne Spenclay (1547) is possibly identical with John Spenlay (1550) and in IGI data for Bishopwearmouth (Durham): Tho. Spenley (1648) and Richard Spencley (1651).
The same developments occurred outside N England. Compare Henry Spendelowe 1309 in Subsidy Rolls (Beds) with William Spendley 1569, Thomas Spenley 1571, and John Spensly 1611 in IGI (Bedford Beds).