Impulsive to dam up lid (6)
I believe the answer is:
madcap
'impulsive' is the definition.
(synonyms)
'dam up lid' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'lid' becomes 'cap' (I've seen this before).
'dam' in reverse letter order is 'mad'.
'mad'+'cap'='MADCAP'
'to' acts as a link.
(Other definitions for madcap that I've seen before include "Amusingly eccentric" , "Reckless, hot-headed" , "Hare-brained" , "terror" , "Given to reckless behaviour" .)