A cot finally brought into play -- bed (7)
I believe the answer is:
stratum
'bed' is the definition.
(I've seen this before)
'a cot finally brought into play' is the wordplay.
'finally' indicates one should take the final letters.
'brought into' is an insertion indicator.
'play' becomes 'strum' (I've seen this before).
The final letter of 'cot' is 't'.
'a'+'t'='at'
'at' put within 'strum' is 'STRATUM'.
(Other definitions for stratum that I've seen before include "(Geological) layer" , "Layer (of rock)" , "rock band" , "stretch of rock" , "Social class - level" .)