Old structure on top of mosque, say - attempt measure of distance (8)
I believe the answer is:
odometry
'measure of distance' is the definition.
The definition suggests a singular noun which matches the answer.
'old structure on top of mosque say attempt' is the wordplay.
'old' becomes 'o' (abbreviation).
'structure on top of mosque say' becomes 'dome' (I can't explain this - if you can you should give a lot more credence to this answer).
'attempt' becomes 'try' (synonyms).
'o'+'dome'+'try'='ODOMETRY'
Can you help me to learn more?
(Another definition for odometry that I've seen is " Method of estimating distance travelled".)