Excessively small amount constrains old male (3,4)
I believe the answer is:
too much
'excessively' is the definition.
(synonyms)
'small amount constrains old male' is the wordplay.
'small amount' becomes 'touch' (touch is a kind of amount. I am not sure about the 'small' bit.).
'constrains' indicates putting letters inside.
'old' becomes 'o' (abbreviation).
'male' becomes 'm' (common abbreviation).
'o'+'m'='om'
'touch' placed around 'om' is 'TOO MUCH'.
Can you help me to learn more?
(Other definitions for too much that I've seen before include "American expression of approval" , "See 17" , "More than enough" , "Excess, more than is needed" , "Excessive, overwhelming" .)