Length of time in the lake? (5)
I believe the answer is:
metre
'length' is the definition.
(I've seen this before)
'time in the lake?' is the wordplay.
'time' becomes 't' (abbreviation).
'in the' means one lot of letters goes inside another.
'lake?' becomes 'mere' (mere is a kind of lake).
't' inserted into 'mere' is 'METRE'.
'of' acts as a link.
(Other definitions for metre that I've seen before include "measure of length" , "100cm" , "three and a bit of a fourth" , "Moisture remover" , "Unit of length in metric system" .)