Constant thundering and in Lancaster maybe small downpour (10)
I believe the answer is:
cloudburst
'small downpour' is the definition.
'cloudburst' can be an answer for 'downpour' (synonyms). I'm unsure of the 'small' bit.
'constant thundering and in lancaster' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'thundering' becomes 'loud' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'in lancaster' becomes 'burst' (I am not sure about this - if you are sure you should believe this answer much more).
'c'+'loud'+'burst'='CLOUDBURST'
'maybe' is the link.
Can you help me to learn more?
(Other definitions for cloudburst that I've seen before include "Sudden, heavy downpour" , "storm" , "Sudden rainstorm" , "dramatic downfall" , "sudden shower" .)